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FORECASTS IN URBAN TRANSPORTATION PLANNING: USES, M-ETHODS~ AND DILEMMAS MARTIN WACHS Graduate School of Architecture ~ Urban Planning, University of California, Los Angeles, CA 90024. Abstract. Legislation and regulation require cities to prepare forecasts of patronage and cost when they compete for federal funds with which to build urban rapid transit systems. Experience shows that these forecasts routinely overestimate patronage and underestimate costs. The explanation for this phenomenon is to be found not in technical shortcomings in the forecasting methods, but in the fact that the forecasts are critically dependent on as- sumptions of key parameters. Assumptions axe chosen to produce forecasts which justify projects favored on the basis of local politi cs. In an environment in which competition for funds is vigorous, effective advocacy becomes more important than objectivity in the use of forecasting methods. This paper reviews forecasting models widely used by transportation consultants, and shows how the models are manipulated in order to promote systems which have been chosen on the basis of political criteria. 1. Introduction During the last twenty years, the single most important change in American trans- portation policy has been an increased commitment to public transit. Federal, state, and local subsidies to support the operation and maintenance of transit systems grew from a total of $88 million in 1965 to more than $7.8 billion in 1980. (Pucher and Markstedt, 1983). While most of these funds have been used to expand, modernize, and operate existing bus systems, a more controversial part of the program has been the construction and modernization of rail rapid transit systems. Under federal law, the Urban Mass Transportation Administration (UMTA), part of the U.S. Department of Transportation, is authorized to provide grants covering up to 75 percent of the cap- ital cost of major transportation projects, including the construction of new subways, provided that local and state governments cover the remainder. In fiscal year 1984, the federal government alone appropriated about a billion dollars for the rail program, and nearly half of this money was allocated for the construction of new systems in cities which did not already have them (Passenger Transport, 1984). New rail transit systems are already operating in San Francisco, Washington, D.C., Atlanta, Baltimore, and Mi- ami. Detailed engineering studies are underway for the construction of a rail line in Los Angeles. Climatic Change n (1987) 61 80. 9 1987 by D. Reidel Pu blishing Company,

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Page 1: Forecasts in Urban Tranportation Planning

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F O R E C A S T S I N U R B A N T R A N S P O R T A T I O N P L A N N I N G :

U S E S , M - E T H O D S ~ A N D D I L E M M A S

M A R T I N W A C H S

Graduate School of Architecture ~ Urb an Pla nnin g,

Univers i ty o f California, Los Ange les , C A 90024.

A b s t r a c t . L e g i s la t io n a n d r e g u l a ti o n r e q u ir e c it ie s t o p r e p a r e f or e c as ts o fp a t r o n a g e a n d c o s t w h e n t h e y c o m p e t e f o r f e d e r a l f u n d s w i t h w h i c h t o b u i l d

u r b a n r a p i d t r a n s i t s y s t e m s . E x p e r i e n c e s h o w s t h a t t h e s e f o r e c a s ts r o u t i n e l y

o v e r e s t i m a t e p a t r o n a g e a n d u n d e r e s t i m a t e c o s ts . T h e e x p l a n a t i o n f o r t h i sp h e n o m e n o n i s t o b e f o u n d n o t i n t e c h n i c a l s h o r t c o m i n g s i n t h e f o r e c a s t i n gm e t h o d s , b u t i n t h e fa c t t h a t t h e f o r e c a s ts a r e c r it i c a l l y d e p e n d e n t o n a s -

s u m p t i o n s o f k e y p a r a m e t e r s . A s s u m p t i o n s ax e c h o s e n to p r o d u c e f o r e c a s tsw h i c h j u s t i f y p r o j e c t s fa v o r e d o n t h e b a s i s o f l o c a l p o l i t ic s . I n a n e n v i r o n m e n t

i n w h i c h c o m p e t i t i o n f o r f u n d s i s v i g o ro u s , e f fe c ti v e a d v o c a c y b e c o m e s m o r e

i m p o r t a n t t h a n o b j e c t i v i t y i n t h e u s e o f f o r e c a s t i n g m e t h o d s . T h i s p a p e r

r e v i e w s f o r e c a s t i n g m o d e l s w i d e l y u s e d b y t r a n s p o r t a t i o n c o n s u l t a n t s , a n d

s h o w s h o w t h e m o d e l s a r e m a n i p u l a t e d i n o r d e r t o p r o m o t e s y s t e m s w h i c h

have be en cho sen on the b as i s o f po l i t i ca l c r i t e r i a .

1 . I n t r o d u c t i o n

D u r i n g t h e l a s t t w e n t y y e a r s , th e s i n gl e m o s t i m p o r t a n t c h a n g e i n A m e r i c a n t r a n s -

p o r t a t i o n p o l i c y h a s b e e n a n i n c r e a s e d c o m m i t m e n t t o p u b l i c t r a n s i t . F e d e r a l , s t a t e ,

a n d l o c a l s u b s i d i e s t o s u p p o r t t h e o p e r a t i o n a n d m a i n t e n a n c e o f t r a n s i t s y s t e m s g r e w

f r o m a t o t a l o f $ 88 m i l l i o n i n 1 96 5 to m o r e t h a n $ 7 .8 b i l li o n i n 1 98 0 . ( P u c h e r a n d

M a r k s t e d t , 1 9 83 ). W h i l e m o s t o f t h e s e f u n d s h a ve b e e n u s ed t o e x p a n d , m o d e r n i z e ,

a n d o p e r a t e e x i s t i n g b u s s y s t e m s , a m o r e c o n t r o v e r s i a l p a r t o f t h e p r o g r a m h a s b e e n

t h e c o n s t r u c t i o n a n d m o d e r n i z a t i o n o f r a i l r a p i d t r a n s i t s y s t e m s . U n d e r f e d e ra l l a w ,

t h e U r b a n M a s s T r a n s p o r t a t i o n A d m i n i s t r a t i o n ( U M T A ) , p a r t o f t h e U .S . D e p a r t m e n t

o f T r a n s p o r t a t i o n , i s a u t h o r i z e d t o p r o v i d e g r a n t s c o v e r in g u p t o 7 5 p e r c e n t o f t h e c a p -

i t a l c o s t o f m a j o r t r a n s p o r t a t i o n p r o j e c t s , i n c l u d i n g t h e c o n s t r u c t i o n o f n e w s u b w a y s ,

p r o v i d e d t h a t l o c a l a n d s t a t e g o v e r n m e n t s c o v e r th e r e m a i n d e r . I n f i sc a l y e a r 1 9 84 , t h e

f e d e r a l g o v e r n m e n t a l o n e a p p r o p r i a t e d a b o u t a b i ll i o n d o l l a r s f or t h e r a i l p r o g r a m , a n d

n e a r l y h a l f o f t h i s m o n e y w a s a l l o c a t e d fo r t h e c o n s t r u c t i o n o f n e w s y s t e m s i n c it ie s

w h i c h d i d n o t a l r e a d y h a v e t h e m ( P a s s e n g e r T r a n s p o r t , 1 9 84 ). N e w r a i l t r a n s i t s y s t e m s

a r e a l r e a d y o p e r a t i n g i n S a n F r a n c i s c o , W a s h i n g t o n , D . C . , A t l a n t a , B a l t i m o r e , a n d M i -

a m i . D e t a i l e d e n g i n e e r i n g s t u d i e s a r e u n d e r w a y f o r th e c o n s t r u c t i o n o f a r a i l li n e i n L o s

A n g e l e s .

Climatic Change n (1987) 61 80.

9 1987 by D. Reidel Publishing Company,

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62 MARTIN WACHS

F o r e c a s t i n g t h e c o s t a n d p a t r o n a g e o f p r o p o s e d t r a n s i t s y s t e m s p l a y s a si g n if i ca n t

p a r t i n t h e a l l o c a t i o n o f f e d e r a l s u b s i d i es f o r t h e i r c o n s t r u c t i o n . F o r e c a s t i n g i s c r i t ic a l

b e c a u s e t h e r a i l p r o g r a m i s , t o a g r e a t e r e x t e n t t h a n o t h e r t r a n s i t s u b s i d y p r o g r a m s ,

c o n s i d e r e d t o b e a " d i s c r e t i o n a r y " p r o g r a m . W h i le t r a n s i t s u b s i di e s fo r t h e o p e r a t i o n

o f e x i s t in g s y s t e m s a r e d i s t r i b u t e d t o c i ti es o n t h e b a s i s o f a f o r m u l a w h i c h t a k e s i n t o

c o n s i d e r a t i o n s u c h v a r i a b l e s a s p o p u l a t i o n , p o p u l a t i o n d e n s i t y , a n d v e h i c l e m i l e s o f

t r a n s i t s e r vi c e , g r a n t s f o r t h e c o n s t r u c t i o n o f n e w s y s t e m s a r e a w a r d e d t o c i ti e s o n t h e

b a s i s o f c o m p e t i n g a p p l i c a ti o n s i n c o r p o r a t i n g s u p p o s e d l y r i g o r o u s e v a l u a ti o n s o f t h e

c o s t - e f f e c ti v e n e s s o f p a r t i c u l a r p r o p o s a l s . C o m p e t i t i o n i s b r i s k a n d p o l i t i c a l s t a k e s a r e

h i g h , a s m a n y c i t i e s a t t e m p t t o g e t u p t o t h r e e f e d e r a l d o l l a r s f o r e a c h l o c a l d o l l a r t h a t

t h e y s p e n d o n a n e w t ra n s i t s y s t em . E a c h c it y m u s t d e m o n s t r a t e t h a t t h e p a r t ic u l a r

s y s t e m i t is r e c o m m e n d i n g i s t h e m o s t c o s t - ef f ec t iv e c h o ic e a m o n g t h e a l t e r n a ti v e s i t h a s

c o n s i d e r e d , a n d t h a t i ts p r e f e r re d s y s t e m is c o s t- e f fe c t iv e i n c o m p a r i s o n w i t h t h e f a v o r e d

p r o j e c t s o f o t h e r c i ti es . F o r e c a s t s o f f u t u r e p a t r o n a g e a n d c o s t p r o v i d e th e p r i n c i p a l

f o r m a l e v i d e n c e u s e d t o e v a l u a t e t h e a l t e r n a t i v e s , a n d t o j u s t i f y f e d e r a l d e c i s io n s t o

f u n d c e r t a i n p r o j e c t s a n d d e f e r o t h e r s . T h e f o r e c a s ts a r e p r e p a r e d i n c o m p l i a n c e w i t h a

f e d e ra l l y - m a n d a t e d e v a l u a t i o n p r o c e d u r e , c a ll e d " a l t e r n a t i v e s a n a l ys i s, " a n d a r e m o s t

o f t e n c a r r i e d o u t b y c o n s u l t i n g f i r m s w h i c h s p e c i a li z e i n f o r e c a s t i n g t r a v e l p a t t e r n s a n d

i n t r a n s p o r t a t i o n f in a n c e. T h e fo r e c a s t in g m e t h o d s w h i c h t h e y e m p l o y h av e b e c o m e

s t a n d a r d i z e d , a n d h a v e b e e n a p p l i e d o v e r t h e p a s t t h i r t y y e a r s t o h i g h w a y a s w el l a s

t r a n s i t p r o g r a m s a l l o v e r t h e w o r l d .

2 . C o m p a r i s o n s o f f o r e c a s t s w i t h a c t u a l p a t r o n a g e a n d c o s t

T h e M i a m i s u b w a y w a s f o r e c a s t t o s e r v e o v e r 1 0 0 , 0 0 0 p a s s e n g e r s p e r d a y w h e n a p p l i -

c a t i o n s w e r e m a d e f o r f e d e ra l f u n d i n g . T o d a y t h e i n c o m p l e t e s y s t e m , w i t h a ll o f i t s

b u s f e e d e r r o u t e s n o t y e t i n p l a c e , i s a c t u a l l y c a r r y i n g a b o u t 9 , 0 0 0 p a s s e n g e r s p e r d a y .

T h e s y s t e m ' s p l a n n e r s , a r e n o w e s t i m a t i n g t h a t r i d e r sh i p w il l " u l t i m a t e l y " r e a c h a b o u t

4 0 , 00 0 p a s s e n g e r s p e r d a y ( D a v i d s o n , 1 9 84 ). M a n a g e r s o f t h e n e w B a l t i m o r e s u b w a y

s t a t e d i n a p u b l i s h e d i n t e r v ie w t h a t t h e y n o w e x p e c t a n n u a l d a i l y p a t r o n a g e o f 3 0 ,0 0 0,

p e r h a p s g r o w i n g o v er s e v e ra l y e a rs t o 45 ,0 0 0. E a r l i er , h o w e v e r , t e c h n i c al d o c u m e n t sw h i c h r e c o m m e n d e d t h e p r o j e c t t o f e d e ra l a u t h o r i t i e s h a d e s t i m a t e d p a t r o n a g e b e t w e e n

8 0 , 0 0 0 a n d 9 0 , 0 0 0 t r a v e l e r s p e r d a y ( H i l l , 1 9 8 3 ) .

P l a n n e r s i n L o s A n g e l e s a r e p r o p o s i n g t o b u il d a n 1 8 -m i le s u b w a y w h i c h t h e i r p a -

t r o n a g e e s t i m a t e s c l a i m w i ll s e rv e s o m e 3 6 4 ,0 0 0 p a ss e n g e r s p e r d a y a n d t h e y t a k e p a i n s

t o e m p h a s i z e t h a t t h e s e e s ti m a t e s a r e c o n si d e r e d t o b e " e x t r e m e l y c o n s e r v a ti v e . " T h e

Los Angeles Times a s k e d a n i n d e p e n d e n t e x p e r t o n p a t r o n a g e f o r e c a s ti n g t o r e v i ew t h e

e s t i m a t e s , a n d h e c o n c l u d e d t h a t t h e f o r e c a st s a r e " o v e r s t a t e d b y a t l e a s t 2 0 p e r c e n t ."

( T r o m b l y , 1 9 8 4) . A n d a t e c h n ic a l s t u d y b y A r t h u r D . L i t t le a n d A s s o c i a t e s ( 1 9 8 2) c o n -

c l u d e d t h a t a p r o p o s e d J a p a n e s e - s t y l e b u l l e t t r a i n s e rv i c e b e t w e e n L o s A n g e l e s a n d S a nD i e g o m i g h t b e e x p e c t e d t o c a p t u r e 4 6 p e r c e n t o f a ll th e b u s i n e s s t r i p s a n d 2 7 p e r c e n t

o f a l l t h e n o n - b u s i n e s s t r i p s b e t w e e n t h e t w o c i ti e s , d e s p i t e t h e f a c t t h a t t h e C a l i fo r -

n i a D e p a r t m e n t o f T r a n s p o r t a t i o n h a d e a r l ie r s p o n s o r e d a s t u d y i n w h i c h c o n s u l t a n t s

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F O R E C A S T S I N U R B A N T R A N S P O R T A T I O N P L A N N I N G : U S E S , M E T H O D S , A N D D I L E M M A S 63

had estimated the potential patronage for a high speed train between the two cities

to be approximately 1.9 percent of all the trips made between them (Barton-Aschman

Associates, 1976).

These most recent examples are quite typical of experience gathered over twenty

years or more in the transit industry. I have examined older technical reports in which

patronage forecasts were made for subways prior to their construction. Adjusting these

forecasts for deletions of proposed lines from the systems, and comparing them with

actual reported ridership in the years for which the forecasts were made, I found a

consistent pattern. The Bay Area Rapid Transit Sys tem (BART) was carrying 140,000

passengers on an average weekday in a year for which it was forecast to carry 264,000;

the Washington Metro served 230,000 people in a year for which it was forecast to carry

440,000; and Atlanta' s system provided service to a daily average of 34,000 during a yearin which it was forecast to serve 110,000 people per day (Parson, Brinkerhoff-Tudor-

Bechtel, 1971; Parsons, Brinkerhoff-Tudor-Bechtel, Inc., e t a l . , 1962; Peat, Marwick,

Mitchell & Co., 1962; Simpson and Curtin, 1962; Urban Mass Transportation Admin-

istration, 1982a and 1982b; Washington Metropolitan Transportation Authority, 1978;

Dunphy and Griffiths, 1981; Parsons, Brinkerhoff, Quade and Douglas, 1962; American

Public Transit Association, 1981a and b; American Public Transit Association, 1980).

Exacerbating the problem of patronage forecasting has been the prevalence of cost

overruns. Capital costs of construction and rolling stock for the Atlanta, Washington,

and San Francisco systems (in constant dollars corrected for inflation) were actually

between 132 and 221 percent of forecast capital costs. And annual opera ting costs

for labor, maintenance, and electric power ranged from 177 percent to 615 percent of

forecasted values (American Public Trans it Association, 1982; American Public Transit

Association, 1980; Urban Mass Transportation Administration, 1982a and b; American

Public Transit Association, 1981a and b).

Patronage overestimates and cost underestimates have combined to make the cost

per passenger carried as much as four to twelve times the forecast values. New local

property and sales taxes, the subvention of bridge toll revenues, and increased fares have

all been required, as actual conditions deviate further from forecast values. Despite

this, those who propose to build new rail transit systems continue to offer optimistic

statements about future ridership and cost.

3 . Forecast errors or del iberate dis tor t ions

There are two explanations for the situation I have described. It could be argued tha t

forecasting is inherently inexact, and that the observed errors result from imperfect

techniques. Perhaps planners have had inadequate experience with transpor tation costs

and pa tronage to provide the basis for more accurate forecasts. The second possibleexplanation is tha t travel and cost forecasting is deliberately slanted to produce figures

which constitute technical justification for public works programs favored on the basis

of political rather than economic or technical criteria. Perhaps vigorous competition

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64 MARTIN WACHS

for funds, and the requirements that politically determined choices be supported by

technical evidence, lead to success in competition for federal funds onl~ if forecasts of

patron age are exaggerated and forecasts of cost are outrageously optimistic.

The second hypothesis seems to be the more clearly supported by the weight of ev-idence. Forecasted patronage is always apparently higher than actual patronage while

forecasted costs always seem to be lower th an actu al costs. It would be reasona ble to

expect that errors arising from flawed methods or inexact procedures would be suffi-

ciently ran dom th at they might at least occasionally fall in the other direction. And,

if costs have bee n unde resti mate d and p atronage overestimated for decades, one would

expect that systematic errors in the methodology might by now have been recognized

and addressed through the refinement of the st anda rd techniques.

The bi ographer of Robert Moses provides us with anecdot al evidence tha t planne rs

knowingl y produc e self-serving and disto rted forecasts. Moses was responsible for many

of the bridges~ tunnels, and parkways in the New York metropolitan are% and his

biographer has written:

cOnce you sink that first stake, ' he would often say, ~theyql never make you

pull it u p' .. . If ends justified means and if the important thing in building

a project was to get it started, than then any means that got it started were

justified. Furnishing misleading information about it was justified; so was

underestimating its costs.

Misleading and underestimating, in fact might be the only way to geta project started. Since his projects were unprecedentedly vast, one of the

biggest difficulties getting them started was the fear of public officials...

tha t the state couldn' t afford the projects (which}... would drain off a share

of the state 's wealth incommensurate with their benefits.

But what if you didn't tell the officials how much the projects would

cost? What if you let the legislators know about only a fraction of what

you knew would be the project's ultimate expense?

Once they had authorized that small init ial expenditure and you had

spent, they would not be able to avoid giving you the rest when you asked

for it. How could they? If they refused to give you the rest of the money,

what they had given you would be wasted, and that would make them look

bad in the eyes of the public. And if they had been misled, that would

mean they hadn't investigated the projects thoroughly, and had therefore

been derelict in their own duty. The possibilities for a polite but effective

form of political blackmail were endless (Caro, 1975, pp. 218-19).

In my view this quotation captures the essence of the ways in which forecasts

are used in the pl anni ng of politically sensitive rail transit systems. Unfortunat ely,

policym akers and m embe rs of the lay public are sometimes far more aware of the political

nat ure of forecasting than are the engineers, economists and pla nners who profess to be

expert in the field.

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FORECASTS IN URBAN TRANSPORTATIONPLANNING:USES , METH ODS, AND DILEMMAS 65

4 . H o w f o r e c a s t s b e c o m e b i a s e d

W i l l i a m A s c h e r ( 1 9 7 8 ) h a s p o i n t e d o u t t h a t f or e c a s t i n g i s c ri ti ca ll y e p e n d e n t o n t h e u s e

o f a s s u m p t i o n s . H e s t u d i e d t h e a c c u r a c y o f f o r e c a s t s m a d e o v e r a p e r i o d o f f if ty e a r s i n

t h e f i el d s f p o p u l a t i o n , e c o n o m i c , e n e r g y , a n d t e c h n o l o g i c a l f o r e c a s t in g , a n d c o n c l u d e d

t h a t % o r e a s s u m p t i o n s " w e r e m o r e i m p o r t a n t d e t e r m i n a n t s o f f o r e c a s t a c c u r a c y t h a n

t h e m e t h o d s u s e d i n f o r e c a s t in g . H e w r o t e t h at :

T h e c o r e a s s u m p t i o n s u n d e r l y i n g a f o re c as t , h i c h r e p r e s e n t t h e f or e ca s te r' s

b a s i c o u t l o o k o n t h e c o n t e x t w i t h i n w h i c h t h e s p ec if i c o r e c a s t e d t r e n d d e -

v e l o ps , a r e t h e m a j o r d e t e r m i n a n t s o f f o r e c as t a c c u r a c y . M e t h o d o l o g i e s a r e

b a s ic a l ly t h e v e h i c l e s f o r d e t e r m i n i n g t h e c o n s e q u e n c e s o r i m p l i c a t i o n s o f

c o r e as s u m p t i o n s t h a t ha v e b e e n c h o s e n m o r e o r l e ss i n d e p e n d e n t ly o f t h e

s pe ci fi c e t h o d o l o g i e s . W h e n t h e c o r e a s s u m p t i o n s a r e v a li d , h e c h o i c e o f

m e t h o d o l o g y is e i th e r e c o n d a r y o r o b v i o us . W h e n t h e c o re a s s u m p t i o n s f ail

t o c a p t u r e t h e r e al i ty f t h e f u t u r e c o n t e x t , o t h e r f a c to r s s u c h a s m e t h o d o l -

o g y g e n e r a l l y a k e l it tl e i ff er en c e; h e y c a n n o t g s a v e " t h e f o re c a s t ( A s h e r ,

1978, p. 199).

I n m y o p i n i o n , th e c o m p e t i t i v e , p o l i t i c a l ly c h a r g e d e n v i r o n m e n t o f t r a n s p o r t a t i o n

f o r e c a s t i n g h a s r e s u l t e d in t h e c o n t i n u o u s a d j u s t m e n t o f a s s u m p t i o n s u n t i l t h e y p r o d u c e

f o r ec a s ts w h i ch s u p p o r t p o l i ti c a l l y a t t r a c t i v e o u tc o m e s . T h e m a t h e m a t i c a l m o d e l s u s ed

a r e c o m p l e x , a n d t h i s c o m p l e x i t y o b s c u r e s t h e f a c t t h a t t h e f o r e c a s ts a r e m o r e c r i t ic a l l yd e p e n d e n t o n a s s u m p t i o n s t h a n t h e y a r e o n th e m a t h e m a t i c a l m a n i p u l a t i o n s w h i ch

d o m i n a t e t e c h n i c a l r e p o r t s a n d b r i ef i ng s . C i t i z e n s a r e t o l d t h a t " s c ie n ti fi c " m o d e l s

w e r e u s e d to e s t i m a t e t h e c o st a n d p a t r o n a g e o f f u t u r e t r a n s i t s y s te m s . T h e c o m p l e x i t y

o f t h e m o d e l s , h o w e v e r , m e r e l y d i s g u i s e s t h e fa c t t h a t a s s u m p t i o n s a n d p a r a m e t e r s a r e

c o n t i n u a l l y a d j u s t e d u n t i l t h e i n t e n d e d c h o i c e i s j u s ti f i ed b y t h e f o r ec a s ts .

M a i n t a i n i n g a n i l lu s i o n f t e c h n i c a l o b j e ct i v i t y b y p l a c in g c o m p u t e r i z e d f o r e c a s t s o f

t r a v e l d e m a n d a n d c o s t i n a c e n t r a l p o s i t i o n i n p o l i c y d eb a t e s , t r a n s i t o ff ic ia ls o l i t e ly

d i s g u i s e t h e f a c t t h a t a l l o c a t i o n s o f bi l l io n s o f d o l l a r s i n g r a n t s f o r n e w t r a n s i t s y s t e m s

a r e p o l i ti c a l ly m o t i v a t e d . L o c a l o f f ic ia ls i n o r l o s e e l e c t i o n s o n t h e b a s i s o f t h e i r

s u c c e s s o r f a i l u r e i n g a r n e r i n g f e d e r a l f u n d s f o r f a v o r e d p r o j e c t s , a n d i t s e e m s i n e v i t a b l e

t h a t p o l it i ca l s a l i en c e , r a t h e r t h a n t e c h n i c a l o bj e c ti v i ty , g o v e r n s t h e e s t i m a t e s o f f u t u r e

t r av e l d e m a n d a n d c o s t p r o d u c e d b y t h e i r c o n s u lt a n t s . A l te r n a t iv e s a n a l y s i s i n t w o

c it ie s h a v i n g s i mi l a r u r b a n f o r m , a u t o m o b i l e o w n e r s h i p , a n d c o m m u t i n g p a t t e r n s , o f t e n

r e v e a l s u r p r i s i n g d i ff e re n ce s . A s u b w a y i s f o u n d t o b e m o r e c o s t e f fe c ti v e i n o n e , w h e r e

t h e b u s i n e s s c o m m u n i t y h a s l o n g b e e n b a c k i n g a s u b w a y , w h i l e p ar a ll e l s t ud i e s " p r o v e "

t h a t e x p r e s s b u s w a y s a r e m o r e j u st i f i e d i n t h e o t h e r c it y , w h e r e p u b l i c l e a d e r s h a v e

r e a c h e d a c o n s e n s u s i n s u p p o r t o f t h a t a l t er n at i ve .

T h e f o l l o w i n g s e c t i o n s o f t h i s p a p e r w i l l p r e s e n t t h e c h a r a ct e r i s t ic s f t r a v e l d e m a n d

a n d t r a n s po r t a t io n c o s t f o r e c a s t in g m o d e l s , a n d e x a m i n e t h e w a y s i n w h i c h t h es e m o d e l s

a n d t h e i r c o r e a s s u m p t i o n s c a n b e u s e d t o p r o d u c e f o r e c as t s i n s u p p o r t o f p ol it ic al

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6 6 M A R T I N W A C H S

o b j e c ti v e s. I n t h e n e x t s e c t io n , t ra v e l d e m a n d f o r e c a s ti n g m o d e l s a re e x a m i n e d , a n d

t h e n t r a n s p o r t a t i o n c a p i t a l a n d o p e r a t i n g c o s t f o r e c a s ts a r e d i s c us s e d . T h e f in a l s e c t io n

o f t h e p a p e r is a n e x a m i n a t i o n o f t h e e t h i c a l d i l e m m a s f a c e d b y f o r e c a st e r s w h o p r o d u c e

t h e e s t i m a t e s w h i c h a r e u s e d i n p o l i t i c a l l y c h a r g e d s i t u a t i o n s .

5 . T r a v e l d e m a n d f o r e c a s t i n g t e c h n i q u e s

T r a v e l d e m a n d f o r e c a s t in g i s d o n e u s i n g a s e t o f c o m p u t e r m o d e l s a n d p r o c e d u r e s w h i c h

a r e h i g h l y i n s t i t u t i o n a l i z e d . S i m i l a r fo r e c a s t i n g m o d e l s h a v e b e e n a p p l i e d o v e r a p e r i o d

o f t h i r t y o r m o r e y e a r s t o c it i e s a s d i v e r s e a s N e w Y o r k , L o s A n g e l e s , L a g o s , H o n g

K o n g , a n d C a l c u t ta . T h e e x t e n t t o w h i c h th e s e m e t h o d s h a v e b e c o m e s t a n d a r d i z e d i s

i n d i c a t e d b y th e f a c t t h a t t h e U . S . D e p a r t m e n t o f T r a n s p o r t a t i o n w i d e l y d i s t r ib u t e s

c o m p u t e r t a p e s a n d m a n u a l s f o r t h e m o d e l s , p u b li s h e s r e g u l a r n e w s l e t t e r s r e g a r d i n g

t h e i r u s e , h o l d s p e r i o d i c t r a i n i n g s e s s io n s a t a v a r i e t y o f l o c a t i o n s , a n d e v e n o p e r a t e s

a t e l e p h o n e h o t l i n e w h i c h u s e r s c a n c a l l if t h e y a r e e x p e r i e n c i n g d i f f ic u l t y u s in g t h e

m o d e l s .

5 . 1 T h e d a t a b a s e

T h e d a t a b a s e f o r t r a v e l f o r e c a s ti n g c o n si s ts o f t h r e e m a j o r c o m p o n e n t s . F i r s t , t h e r e

i s a d e t a i le d i n v e n t o r y o f t r a v el , u s u a l l y o b t a i n e d b y h o m e i n t e r v ie w s s u p p l e m e n t e d

b y r o a d s i d e i n t e r v i e w s o f a s a m p l e o f t r a v e l er s . T h e s u r v e y s , r e f e r r e d t o a s " o r i g i n -

d e s t i n a t i o n " s t u d ie s , r e c o r d t h e o r i gi n , d e s t i n a t i o n , p u r p o s e , m o d e o f t r a v e l ( i . e . , a u t o ,

b u s , e t c . ) , a n d t i m e o f t r a v e l f o r al l t r i p s m a d e b y a ll m e m b e r s o f t h e s u r v e y e d h o u s e h o l d s

o n t h e s u r v e y da y . I n a d d it i o n , s o c i a l a n d d e m o g r a p h i c d a t a a r e co l l ec t e d s o t h a t

a n a l y s t s c a n r e l a t e tr a v e l p a t t e r n s t o h o u s e h o l d c h a r a c t er i s ti c s . F o r ex a m p l e , i n L o s

A n g e l e s d e t a i le d t r a v e l d a t a w e r e co l l ec t e d f r o m a s a m p l e o f m o r e t h a n 1 4 ,0 0 0 h o u s e h o ld s

i n 1 9 6 1 , a n o t h e r 7 , 0 0 0 h o u s e h o l d s i n 1 9 6 7 , a n d 3 , 5 00 h o u s e h o l d s i n 1 9 76 . I n m a n y c i t i e s,

i n c l u d i n g C h i c a g o , t h e l a t e s t h o u s e h o l d t ra v e l i n v e n t o r y w a s u n d e r t a k e n i n t h e s a m e

y e a r a s t h e U . S . C e n s u s i n o r d e r t o e n h a n c e t h e u s e f u l n es s o f t h e t r a v e l d a t a .

T h e s e c o n d c r i t ic a l p a r t o f t h e d a t a b a s e f o r t ra v e l a n a l y s i s i s s o m e i n d i c a t i o n o f th e

d i s t r i b u t i o n o f s o c ia l a n d e c o n o m i c a c t iv i t ie s i n s pa c e . T h e d e m a n d f o r tr a v e l is d e ri v e d

f r o m t h e p a t t e r n s o f e c o n o m i c a n d s o c ia l a c t i vi t ie s a n d t h e l o c a t i o n s a t w h i c h t h e y t a k e

p l a c e. C h a n g e s i n tr a v e l p a t t e r n s , t h e n , a r e re f l ec t io n s o f c h a n g e s i n th e l o c a t i o n a n d

i n t e n s i t y o f a c t iv i t ie s . F o r e x am p l e , s u b u r b a n i z a t i o n o f r e s id e n c e s a n d e m p l o y m e n t is

o n e o f t h e m a j o r t r e n d s i n f lu e n c in g r e ce n t c h a n g e s i n t r a v e l p a t t e r n s . A c t i v i t i e s a re

m o s t o f t e n r e p r e s e n t e d f o r t r a v e l a n a l y s i s a s d e s c r i p t i o n s o f l a n d u s e , i n c l u d i n g , f o r

e x a m p l e , t h e n u m b e r o f s i n gl e -f a m i l y a n d m u l t i p l e - f am i l y r e s id e n c e s a t a l o c a t io n , o r

t h e n u m b e r o f s q u a r e f e e t o f c o m m e r c i a l a c t i v i t y o f t h e n u m b e r o f e m p l o y e e s a t b u s i n e s s

e s t a b l i s h m e n t s .

B o t h t r a v e l i n v e n to r i e s ( f r o m o r i g i n - d e s t in a t i o n s t u d i e s ) a n d a c t i v i t y i n f o r m a t i o n( f r o m la n d u s e s u r v e y s , as s e s s m e n t r e c o r d s , a n d p e r h a p s a e r ia l p h o t o s ) a r e s u m m a r i z e d

f o r a m e t r o p o l i t a n a r e a a c c o r d i n g t o a s y s t e m o f z o n e s. I n s o m e a r e a s c e n s u s t r a c t s a r e

u s e d , m o r e r a r e l y c i ti e s a r e r e p r e s e n t e d a s a s y s t e m o f g r i d s q u a re s .

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FORECASTS IN URBAN TRANS PORT ATIO N PLANN ING: USES, METHODS, AND DILEMMAS 67

T h e t h i r d e l e m e n t o f t h e t r a n s p o r t a t i o n d a t a b a s e is a c o m p u t e r i z e d r e p r e s e n t a t i o n

o f t h e t r a n s p o r t a t i o n n e t w o r k , c o n n e c t i n g t h e c e n t r o id s o f a ll th e z o n e s t o o n e a n o t h e r

i n a s y s t e m o f l in k s a n d n o d e s w h i c h r e p r e s e n t t h e r e g i o n a l h i g h w a y a n d t r a n s i t n e t w o r k .

C a p a c i t i e s a n d t r a v e l ti m e s a r e a s s o c i a t e d w i t h e a c h l in k i n a c c o r d a n c e w i t h t h e i r a c t u a l

o p e r a t i n g c h a r a c t e r i s t i c s .

5 . ~ T r a v e l d e m a n d f o r e c a s ti n g m o d e l s

U s i n g t h e f o r e g o i n g e l e m e n t s o f t h e t r a v e l d a t a b a s e , m a t h e m a t i c a l m o d e l s o f t r a v e l

a r e d e v e l o p e d w h i c h f i r s t d e s c r i b e c u r r e n t t r a v e l p a t t e r n s a n d w h i c h a r e t h e n u s e d t o

p r e d i c t f u t u r e t r a v e l in t h e r e g io n . B e c a u s e o f t h e c o m p l e x i t y o f t r a v e l p a t t e r n s , a n d

t h e s h e e r n u m b e r o f i t e m s c o n s t i t u t i n g t h e d a t a b a s e , i t is u s u a l t o r e p r e s e n t t r a v e l

u s i n g a s e q u e n c e o f i n d e p e n d e n t m o d e l s . T h e m o d e l s a r e i n d e p e n d e n t i n t h e s en s e

t h a t t h e o u t p u t s o f o n e b e c o m e th e i n p u t s t o t h e n e x t , a n d t h e r e is n o " f e ed b a c k "a m o n g t h e m . T h e m o d e l s a r e s e q u e n ti a l b e c a u s e t h e y a r e a p p li e d i n o r d e r r a t h e r t h a n

s i m u l t a n e o u s ly . F i g u r e 1 s h o w s t h e o r d e ri n g a n d t h e i n t e r c o n n e c t i o n s a m o n g t h e m o d e l s .

F o r a m o r e c o m p l e t e d e s c r i p ti o n o f t h e m o d e l i n g p r o c e s s t h a n is p r o v id e d i n t h e f o ll o w i n g

p a r a g r a p h s , t h e r e a d e r m a y w i s h t o re v i e w t h e b o o k b y S t o p h e r a n d M e y b u r g ( 19 7 5) .

T h e f i r st s t e p i n t h e s e q u e n c e is t h e d e s c r i p t i o n o f c u r r e n t o r f u t u r e l a n d u s e p a t -

t e r n s . W h e n u s e d in f o r e c a s ti n g , l a n d u s e m o d e l s d i s t r i b u t e a n t i c i p a t e d c h a n g e s in

r e s i d e n ti a l p o p u l a t i o n , e m p l o y m e n t , a n d s q u a r e f e et o f r e t a il i n g a n d c o m m e r c i a l a c t i v i-

t ie s . W e l l k n o w n e x a m p l e s o f l a n d u s e o r a c t i v it y m o d e l s i n cl u d e t h e L o w r y M o d e l a n d

t h e E m p i r i c M o d e l , w h i c h h av e b e e n u se d t o e x t e n d p a s t t r e n d s i n u r b a n g r o w t h o v e r

a p e r i o d o f t e n t o t w e n t y y e a r s i n to t h e f u t u r e .

H a v i n g e s t a b l i s h e d a c t i v i t y p a t t e r n s , t h e n u m b e r o f t r i p s w h i c h o r i g in a t e o r e n d

i n a g i v e n z o n e d u r i n g a p a r t i c u l a r t i m e p e r i o d ( e . g . , a d a y o r m o r n i n g r u s h h o u r ) i s

m o d e l e d i n w h a t i s r e f e rr e d t o as " t r i p g e n e r a t i o n " a n d " a t t r a c t i o n " a n a ly s i s. U s u a ll y ,

m u l t i p l e r e g r e s s i o n i s u s e d , r e l a t i n g t h e t r i p s w h i c h o r i g i n a t e i n a z o n e t o i t s r e s i d e n t i a l

l a n d u s e , i n c o m e , a n d f a m i l y c h a r a c t e r i s t i c s ; a n d r e l a t i n g t h e t r i p d e s t i n a t i o n s i n a

g i v en z o n e to t h e l e ve l o f e m p l o y m e n t o r c o m m e r c i a l a c t iv i t y t h e r e . S e p a r a t e l in e a r

r e g r e s s i o n m o d e l s a r e ty p i c a l l y d e v e l o p e d f o r d i f f e re n t t r i p p u r p o s e s ( e . g . , w o r k , s o c i a l ,a n d s h o p p i n g t r i p s ), a n d f o r d i ff e re n t t i m e p e r i o d s ( e . g . , w e e k d a y s , w e e k e n d s ) . T h e

m o d e l s a r e f i t t o k n o w n t r a v e l p a t t e r n s u s i n g t h e d a t a p r o v i d e d b y o r i g i n - d e s t i n a t i o n

s u r v e y s , a n d r e g r e s s i o n c o e f f i c i e n t s a r e a s s u m e d t o h o l d c o n s t a n t o v e r t e n o r t w e n t y

y e a r s so t h a t n e w a c t i v i t y a n d h o u s e h o l d c h a r a c t e r i s t ic s p r o j e c t e d i n la n d u s e m o d e l i n g

c a n b e u s e d t o p r o j e c t f u t u r e t r ip o r ig i n s a n d d e s t i n a t i o n s f o r e a c h z o n e .

T r i p g e n e r a t i o n e s t i m a t e s t h e t o t a l n u m b e r o f t r i p s o r i g i n a t i n g in e a c h z o n e, a n d

t r i p a t t r a c t i o n p r o d u c e s a n e s t im a t e o f t o t a l t r ip s d e s t i n e d t o e a c h z o n e . I n t h e t h i r d

s t e p , a n o t h e r m o d e l i s e m p l o y e d t o d e t e r m i n e h o w m a n y o f t h e t r i p s o r i g i n a t i n g i n a

p a r t i c u l a r z o n e w il l e n d i n e a c h o f t h e o t h e r z o n e s. T h i s s t e p i n t h e m o d e l i n g s e q u e n c ei s c a l le d " t r i p d i s t r i b u t io n , " a n d i t i s u s u a l l y d o n e w i t h a g r a v i t y - t y p e m o d e l w h i c h

d i s t r i b u t e s t r i p s i n d i r e c t p r o p o r t i o n t o t h e a c t i v i t y l e ve l s i n e a c h z o n e a n d i n in v e r s e

p r o p o r t i o n t o t h e tr a v e l t i m e b e t w e e n t h e m . T h e g r a v i t y m o d e l c a n b e " c a l i b r a te d "

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68 MARTIN WACHS

S t e p s i n t h e S e q u e n t i a l T r a v e lD e m a n d M o d e l i n g P r o c e s s R e s u l t s o f t h e S t e p s

U r b a n A c t i v i t y ( L a n d U s e ) lM o d e l s

T r ip G e n e r a t io n a n d IA t t r a c t i o n M o d e l s IIT r ip ModelsDistributi~ I

I~A

M o d e C h o ic e I -M o d e l sI

Traf f iCModelss s i g n m e n t I

Spa t i a l d i s t r i bu t i on o fr e s i d e n t i a l p o p u l a t i o n

a n d e c o n o m i c a c t i v i t i e s .

T h e n u m b e r o f t r i p s w h i c h o r i g i n a t ein e a c h z o n e a n d th e n u m b e r

w h i c h a r e d e s t i n e d f o r e a c h z o n e ,b y p u r p o s e a n d t im e o f d a y .

T h e t r i p i n t e r c h a n g e sb e t w e e n p a r t i c u l a r o r i g i na n d d e s t i n a t i o n z o n e s , b yp u r p o s e a n d t im e o f d a y .

T h e p r o p o r t io n s o f t h e t r ipi n t e r c h a n g e s b e t w e e n o r i g i n s

a n d d e s t i n a t i o n s w h i c h w i l l u s epar t i cu lar modes (auto , bus , t ra in , e tc . )

T h e r o u t e s w h i c h w i l l b e f o l l o w e db y th e t ri p s u s i n g e a c h o f th e m o d e s ;

a n d t h e tr a v e l v o l u m e s o n th e lin k so f t h e t ra n s p o r t a ti o n s y s t e m .

F i g u r e 1 . S u m m a r y o f t h e U r b a n T r av e l D e m a n d M o d e l i n g P r o c es s .

u s i n g d a t a f r o m t h e o r i g i n - d e s t i n a t i o n s u r v e y , a n d t h e e x i s ti n g t r a v e l ti m e s b e t w e e n

t h e z o n e s . C a l i b r a t i o n e s t a b l i s h e s v a l u e s o f c r i t i c a l co e f f ic i e n ts w h i c h a r e t h e n u s e d

t o d i s t r i b u t e t h e f u t u r e t r i p e n d s w h i c h w e r e e s t i m a t e d i n t h e t r i p g e n e r a t i o n a n d

a t t r a c t i o n m o d e l s . A t t h e e n d o f t h e t h i r d s t a g e i n t h e m o d e l s e q u e n c e , t h e n , p l a n n e r s

h a v e a r e p r e s e n t a t i o n o f t h e n u m b e r o f t r ip s , f o r a p a r t i c u l a r t i m e p e r i o d a n d p u r p o s e ,

w h i c h w i ll t a k e p l a c e b e t w e e n e a c h o r ig i n z o n e a n d e a c h d e s t i n a t i o n z o n e . A t t h i s s t a g e

i n t h e s e q u e n c e , t h e r e i s as y e t n o r e p r e s e n t a t i o n o f t h e m o d e s o f t r a v e l b y w h i c h t h e s e

t r i p s w i l l b e m a d e , n o r t h e r o u t e s o f tr a v e l w h i c h t h e y w i l l f ol lo w . T h e p u r p o s e o f t h e

l a s t t w o s t e p s i n t h e f o r e c a s t i n g p r o c e s s i s t o m o d e l t h e s e a s p e c t s o f t r a v e l c h o i c e .

T h e " m o d a l c h o ic e " o r " m o d a l s pl it " m o d e l d i v i d es t h e t o t a l n u m b e r o f t r ip s b e -

t w e e n a n o r i g in z o n e a n d a d e s t i n a t io n z o n e in t o t h e p r o p o r t i o n w h i c h w i ll t ra v e l b y

a u t o , t r a i n , b u s , a n d a n y o t h e r a p p r o p r i a t e m o d e s . A v a r i e ty o f m o d e l s h a v e b e e n u s e dt o a c c o m p l i s h th i s , a l l b a s e d o n t h e p r i n c i p le t h a t p e o p l e c h o o s e a m o n g t r a v e l m o d e s

o n t h e b a s i s o f t h e c h a r a c t e r i s t i c s o f t h e m o d e s , s u c h a s c o s t , tr a v e l t i m e , a n d c o n v e -

n i e n c e , w h i l e t h e s o c ia l a n d e c o n o m i c c h a r a c t e r i s t i c s ( i n c o m e , a u t o o w n e r s h i p , e t c . ) o f

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FORECASTS IN URBAN TRANSPORTATIONPLANNING:USES, METHO DS, AND DILEMMAS 69

t h e p o p u l a t i o n m a y a l so i n fl u e n ce t h e ir c h o ic e s . A m o n g t h e c h o ic e m o d e l s a p p l i e d a t

t h i s s t a g e o f t r a v e l d e m a n d m o d e l i n g a r e a v a r i e ty o f f o r m s o f th e m u l t i n o m i a l l o g it

f u n c t i o n , a n d s e v e r a l o t h e r m a t h e m a t i c a l fo r m s b a s e d o n th e c o n c e p t t h a t t h e t r a v e l e r

w i l l e n g a g e i n s o m e s o r t o f u t i l i ty m a x i m i z i n g b e h a v i o r . B y r e p l i c a t i n g t h e a c t u a l m o d e

c h o ic e s o f t r a v e l e r s w h o p a r t i c i p a t e d i n t h e o r i g i n - d e s t i n a t i o n s u r v e y , m o d e l c o e ff ic ie n ts

a r e d e r i v e d w h i c h a r e t h e n u s e d t o d i v i d e f o r e c a s t f u t u r e t r i p s a m o n g t h e v a r i o u s m o d e s .

T h e f i n a l s t e p i n t h e t r a v e l a n a l y s i s m o d e l p a c k a g e , l a b e l e d " t r a f f ic a s s i g n m e n t , "

r e c o g n iz e s t h a t a u t o u s e r s m a y c h o o s e a v a ri e t y of r o u t e s b e t w e e n a g i v e n o r i g in a n d

d e s t i n a t i o n , a n d t h a t t r a n s i t u s e r s m i g h t h a v e a c h o ic e a m o n g d i f f e r e nt b u s r o u t e s o r

r a i l l i n e s a s w e l l . I n t h i s l a s t s t e p , t h e t r i p s w i t h i n e a c h m o d e a r e f u r t h e r s u b d i v i d e d

i n t o t h o s e w h i c h w i ll u t i li z e e ac h o f t h e m a j o r r o u t e s b e t w e e n o r i g in s a n d d e s t i n a t i o n s .

L e a s t t i m e p a t h a l g o r i t h m s a r e u s e d , th o u g h t h e y a r e s o m e t i m e s m o d i f i e d to re f le c tt r i p c o s t a s w e l l a s t r a v e l t i m e , a n d t o r e f le c t t h e f a c t t h a t t r a v e l t im e s o n e a c h r o u t e

a r e t h e m s e l v e s v a r i a b l e , d e p e n d i n g o n t h e r a t i o o f t r af f ic v o l u m e u s i n g a r o u t e t o i ts

c a p a c i t y . W i t h i n t h e s e a l g o r i t h m s , c o n g e s ti o n o n a r o u t e c a n s lo w tr a v e l a n d c a u s e a

r e a l l o c a t i o n o f t r a f f ic t o d i f f e r e n t r o u t e s .

A t t h e e n d o f t h i s p r o c e s s o f s e q u e n t ia l , i n d e p e n d e n t s t e p s , w e h a v e a r e p r e s e n t a t i o n

o f t r a v e l v o l u m e s o n e a c h r o u t e v i a e a ch m o d e , a n d t h a t o f c o u r s e i s p r e c i s e ly t h e w a y

t h e t r a n s i t p a t r o n a g e e s t i m a t e s d i s c u ss e d a b o v e w e r e a r r i v e d a t.

T h e s e m o d e l s c a n b e a p p l i e d a n d r e a p p l i e d in s e q ue n c e t o t e s t a l t e r n a t i v e a s s u m p -

t i o n s a b o u t f u t u r e g r o w t h o f p o p u l a t i o n a n d e c o n o m i c a c t i v it y ; t o te s t t h e e f f ec ts of

a d d i n g n e w h i g h w a y s a n d n e w t r a n s i t l i n k s i n to t h e o v e r a ll t r a n s p o r t a t i o n s y s t e m , o r

p e r h a p s t o t e s t t h e e f fe c ts o f c h a n g i n g t r a n s i t f a re s a n d a u t o o p e r a t i n g c o s t s d u e t o fu e l

p r i c e i n c r e a s e s . I t m a y t a k e m o n t h s a n d a s t a f f o f a d o z e n p e o p l e o r m o r e t o g e t a s e t

o f m o d e l s " u p a n d r u n n i n g " f o r a p a r t i c u l a r m e t r o p o l i t a n a r e a , a n d t h e r e i s q u i t e a l o t

o f w o r k i n v o lv e d i f a l a r g e n u m b e r o f a l t e r n a t i v e s y s te m ~ i s t o b e t e s t e d .

6 . S o u r c e s o f e r r o r i n t r a v e l d e m a n d m o d e l s

T h e s e q u e n t i a l in d e p e n d e n t m o d e l i n g o f t r a v el d e m a n d e m b o d i e s a p a r a d o x w h i c h is

a p p a r e n t l y a l l t o o c o m m o n i n c o m p u t e r i z e d r e p r e s e n t a t i o n s o f s o c ia l p h e n o m e n a . T h e

s e t o f m o d e l s a p p e a r s t o b e e x t r e m e l y c o m p l e x , i n t h a t i t c o n s i s ts o f h u n d r e d s o f z o n es

f o r w h i c h d o z e n s o f i t e m s o f d a t a a r e m a n i p u l a t e d b y n u m e r o u s e q u a t i o n s i n v o l v i ng

m a n y p a r a m e t e r s . T h i s i s c o m p l e x i t y o f t h e " a c c o u n t i n g v a r ie t y, " h o w e v e r , w h i c h t e n d s

t o m a s k t h e f a c t t h a t t h e s o c i a l a n d e c o n o m i c r e l a t i o n s h i p s i n c o r p o r a t e d i n t h e m o d e l s

a r e a g r o s s o v e r s i m p l i f i c a t i o n o f r e a l i ty . T h e m o d e l s , f o r e x a m p l e , r e p r e s e n t a s s i n g le

i n d e p e n d e n t s t e p s p h e n o m e n a w h i c h a c t u a l l y o c c u r s i m u l ta n e o u s l y , a n d t h e o r d e r i n g

o f t h e s t e p s i s ch o s e n fo r c o m p u t a t i o n a l r e a s o n s r a t h e r t h a n t o r e p l i c a t e a n y r e a l is t i c

i n t e r a c t i o n o f t h e f o r c e s i n v o lv e d in u r b a n d e v e l o p m e n t o r t r a v e l .

T h e n u m e r i c a l c o m p l e x i t y o f th e m o d e l s m a k e s i t d i ff i cu l t f o r p o l i t ic a l d ec i s io n -

m a k e r s o r l a y c i ti z e n s to g r a s p t h e o p e r a t i o n s o f t h e m o d e l s , a n d t h e r e s u l t s w h i c h a r e

p r e s e n t e d i n t e c h n i c a l r e p o r t s a r e v o l u m i n o u s , d r y , a n d r a r e l y r e a d . T h e m o s t c r i t i c a l

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70 MARTIN WACHS

a s s u m p t i o n s a r e f r e q u e n t l y u n r e p o r t e d , t e c h n i c a l ap p e n d i c e s o f t e n o m i t k e y p a r a m e t e r s ,

a n d c o l u m n s o f n u m b e r s a r e p r e s e n t e d i n w h i c h it i s o f t e n di ff ic u lt t o k n o w w h i c h v a lu e s

a r e a s s u m e d , w h i c h a r e r a w d a t a , a n d w h i c h a r e m o d e l re s u lt s . T h e d o g g e d c r it i c o f

d e m a n d f o r e c a s t s w h o w i s h e s t o r e t r a c e a m o d e l e r ' s s t e p s o f t e n f i n d s t h i s a d i f f i c u l t

u n d e r t a k i n g . I t is m y c o n c l u s i o n , a f t e r s t u d y i n g a p p l i c a t io n s o f t h e s e m o d e l s , t h a t t h e

n u m e r i c a l c o m p l e x i t y o f t h e m o d e l s is u s e d t o o b s c u r e t h e f a c t t h a t t h e y u s u a l ly e m -

b o d y d o z e n s o f s i m p l e a s s u m p t i o n s o f q u e s t i o n a b l e v a li d it y . T h e s e a s s u m p t i o n s d r i v e

t h e o u t c o m e s i n d i r e c ti o n s w h i c h s u p p o r t t h e p r e d e t e r m i n e d p o s i t i o n s o f t h e a g e n c i es

p a y i n g f o r th e f o r e c a s t s . I n t h e n e x t p a r a g r a p h s I w i ll s u m m a r i z e a fe w o f t h e w a y s i n

w h i c h t h i s i s a c c o m p l i s h e d i n p r a c t i c e .

6.1 Circularity in land use and travel models

T h e d e m a n d f o r t r a v e l is d e r i v ed f r o m u r b a n a c t i v i ty p a t t e r n s , s o c h a n g e s i n la n d u s e

p a t t e r n s a r e t h e s i n gl e m o s t i m p o r t a n t c h a n g e s w h i c h d e t e r m i n e t h e o u t c o m e s o f t r av e l

f o r e c a s t s . T h i s i s r e f l e c t e d i n t h e f a c t t h a t t h e f i r st s t e p i n t h e m o d e l s e q u e n c e i s t h e

e s t i m a t i o n o f f u t u r e p o p u l a t i o n a n d e m p l o y m e n t . I f p o p u l a t i o n g r o w t h a n d e m p l o y m e n t

e x p a n s i o n a r e l a r g e , i t fo l lo w s t h a t t r a v e l v o l u m e s w i l l g r o w in r e s p o n s e . A r a i l t r a n -

s i t s y s t e m u s u a l l y s e rv e s th e c e n t r a l b u s i n e s s d i st r i ct , a n d t e n d s t o s u p p o r t c e n t r a li z e d

d o w n t o w n g r o w t h , w h il e a p a t t e r n o f d e c e n t r a li z e d ~ u b u r b a n g r o w t h is o f t e n a d e q u a t e l y

s e r v e d b y a u t o s a n d b u s e s . N o t s u r p r i s i n g ly , t r a v e l f o r e c a s t s i n c it i es f a v o r i n g r ai l t r a n s i t

s y s t e m s h a v e b e e n p r e d i c a t e d u p o n e x u b e r a n t p r o j e c t io n s o f c e n t r a l c i t y e m p l o y m e n t

g r o w t h . T h e B A R T p a t r o n a g e f o r e c a s t s , f o r e x a m p l e , w e r e b a s e d o n a f o r e c a s t e m -

p l o y m e n t l e v e l i n t h e c e n t r a l b u s i n e s s d i s t r i c t o f 3 0 7 ,0 0 0 , w h i l e a c t u a l e m p l o y m e n t i n

t h e f o r e c a s t y e a r o n l y r e a c h e d 2 6 0 ,0 0 0. S i m i la r ly , t h e A t l a n t a p a t r o n a g e f o r e c a s ts w e r e

b a s e d o n a p r o j e c t i o n o f 4 0 0 ,0 0 0 d o w n t o w n e m p l o y e e s a n d t h e a c t u a l l e v e l i n t h e f o r e c a s t

y e a r c a m e t o o n l y 3 6 0 , 0 00 ( P a r s o n s , B r i n k e r h o f f , Q u a d e , a n d D o u g l a s , 1 9 6 2 ; P a r s o n s ,

B r i n k e r h o f f - T u d o r - B e c h t e l , 1 9 71 ; P e a t , M a r w i c k , M i t c h e l l & C o . , 1 9 7 1) .

I t is in t e r e s t in g t o n o t e t h a t l a n d u s e i n a m a j o r m e t r o p o l i t a n a r e a i s i ts e lf d e p e n d e n t

o n t h e l e v e l o f a c c e s s i b il i t y t o t h e d o w n t o w n a r e a , a n d a c c e s s i b il i t y t o d o w n t o w n i s af u n c t i o n o f t h e q u a l i t y o f t r a n s p o r t a t i o n s e rv i ce . I t is t h e r e fo r e , n o t u n c o m m o n t h a t l a n d

u s e f o r e c a s t s e m b o d y a s s u m p t i o n s a b o u t t h e l ev e l o f p u b l i c t r a n s i t s e rv i c e w h i c h w i ll

e x i s t i n t h e t a r g e t y e a r . I n t e r e s t i n g l y , a c o n s o r t i u m o f c o n s u l t a n t s w h o i n t h e e a r l y 1 9 7 0 s

d e v e l o p e d a l a n d u s e p l a n f o r c e n t r a l L o s A n g e l e s assumed t h a t t h e r e w o u l d b e a re g i o n a l

r a il r a p i d t r a n s i t s y s t e m i n o p e r a t i o n b y 1 9 90 . T h e i r f o r e c as t s o f d o w n t o w n e m p l o y m e n t

g r o w t h a r e c l e a r l y m u c h h i g h e r t h a n t h e y w o u l d h a v e b e e n h a d t h e y a s s u m e d t h e r e

w o u l d b e n o r a i l a c c e s s t o t h e c e n t r a l c i ty . Y e t , in r e c e n t t r a v e l d e m a n d f o r e c a s t s

t r a n s p o r t a t i o n c o n s u l t a n t s h a v e u s e d th e s e ea r li e r f o r e c a s ts o f d o w n t o w n e m p l o y m e n t

a s t h e b a s i s fo r th e i r p r o j e c t i o n s o f t r a v e l d e m a n d t o a n d f r o m t h e c e n t r a l a r e a . T h i s

o b v i o u s c i r c u l a r i t y o f t h e f o r e c a s t i n g o f l a n d u s e a n d t r a v e l w a s r i d i c u l e d i n a n a n a l y s i s

o f p l a n n i n g i n L o s A n g e l e s b y P e t e r M a r c u s e , w h o w r o t e :

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FORECASTS IN URBAN TRANSPORTATIONPLANNING:USES, METH ODS, AND DILEMMAS 71

H o w d o y o u c a l c u l a t e th e l i k e ly p a t r o n a g e a l o n g a g i v e n c o r r i d o r in 1 99 07

Y o u m a k e a s s u m p t i o n s a b o u t t h e l o c a t i o n a n d i n t e n s i t y o f l a n d u s e s , m o s t

i m p o r t a n t l y r e s i d e n c e s a n d j o b s , in 1 0 90 . B u t h o w d o y o u k n o w t h e l o c a t i o n

a n d i n t e n s i t y o f l a n d u s e s i n 1 09 0. Y o u l o o k a t w h a t t h e C i t y P l a n p r o j e c t s .

H o w d id t h e C i t y P l a n k n o w w h a t t o p r o j e c t ? I t m a d e a s s u m p t i o n s a b o u t

t h e r a t e o f g r o w t h a n d t h e p a t t e r n s o f t r a n s p o r t a t i o n in t h e f u tu r e . A n d

w h a t a s s u m p t i o n s d id i t m a k e a b o u t t r a n s p o r t a t i o n ? W h y , a f ix e d - gu i d ew a y

h igh -cap ac i ty syst em~ o f cou rse ! So the en t i re p rocess is c i rcu l a r : pa t ron ag e

p r o j e c ti o n s d e t e rm i n e t r a n s p o r t a t i o n p la ns ~ t r a n s p o r t a t i o n p l a n s d e t e r m i n e

l a n d u s e p r o j e c t i o n s , l a n d u s e p r o j e c t i o n s d e t e r m i n e p a t r o n a g e p r o j e c t i o n s .

T h e s e q u e n t i a l , i n d e p e n d e n t c h a r a c t e r i s t i c o f t h e m o d e l i n g s e t p r o v i d e s t h e o p p o r -

t u n i t y f o r t h i s c i r c u l a r i ty . I n r e a l i ty , l a n d u s e a n d t r a v e l e v o lv e s i m u l t a n e o u s l y , a n d a t

a n y t i m e t h e r e is a n e q u i l i b r iu m b e t w e e n t r a v e l a n d e m p l o y m e n t . T h e f a c t t h a t t h e e m -

p l o y m e n t a n d t r a v e l m o d e l s a r e in d e p e n d e n t o f o n e a n o t h e r r e q u i r e s c i r c u l a r i t y i n t h e i r

i m p l e m e n t a t i o n , a n d t h i s in v i t es se l f- s e r v in g a s s u m p t i o n s o f th e s o r t w h i c h M a r c u s e

e n u m e r a t e d .

6 .2 A s s u m p t i o n s i n t h e d e t e r m i n a t i o n o f m o d e l p a r a m e t e r s

W h e n a c h a n g e i s m a d e i n on e p a r a m e t e r o f o n e m o d e l i n t h e s e q u e n c e , i t s e ff e ct i s

c a r r i e d th r o u g h t h e s e q u e n c e a l m o s t w i t h o u t n o t i ce . W h a t a p p e a r s t o b e a t e c h n i c a l

a d j u s t m e n t i n a s in g l e m o d e l c a n , h o w e v e r , d r iv e a l l o f t h e s u b s e q u e n t r e s u l t s t o w a r d

a n i n t e n d e d o u t c o m e . B e c a u s e t h e r e a r e d o z e n s o f p a r a m e t e r s a n d s e p a r a t e c a l i b r a t i o n

p r o c e d u r e s f o r t h e d i f f e r e n t m o d e l s i n th e s e q u e n c e , s u c h a c h a n g e i s o f t e n u n n o t i c e d i n

t h e o v e r a l l m o d e l i n g p r o c e s s .

T h e m o d e c h o ic e m o d e l , f o r e x a m p l e , is u s e d t o d e t e r m i n e t h e p r o p o r t i o n s o f t h e

t r i p s b e t w e e n e a c h o r i g in a n d e a c h d e s t i n a t i o n w h i c h w i ll b e m a d e b y r a i l t r a n s i t , b u s ,

a n d a u t o . N a t u r a l l y , t h e s p e e d o f a u t o t r a f f i c o n t h e h i g h w a y n e t w o r k w i l l a f f ec t t h e

p r o p o r t i o n o f t ra v e l e r s c h o o s in g e a c h m o d e . I f h i g h w a y s p e e d s a r e h i g h , a l a r g e r n u m b e r

o f p e o p l e w il l c h o o s e t o d r i v e t h a n w o u l d b e t h e c a s e if h i g h w a y s a r e s lo w a n d c o n g e s t e d .

S i m i l a rl y , i f p a r k i n g i s u n a v a i l a b l e o r e x p e n s i v e a t e m p l o y m e n t c e n t e r s, f e w e r p e o p l e

w i ll d r i v e t h a n w o u l d b e t h e c a s e if a m p l e f re e p a r k i n g w e r e a v a i la b l e . C o n s e q u e n t l y ,

m o s t m o d e c h o ic e m o d e l s u s e t h e s u p p l y o f p a r k i n g s p a c e s , t h e p r i c e s o f p a r k i n g , a n d

a u t o t r af f ic s p e e d s a s p r e d i c t o r s o f t h e s h a r e s o f t r i p s u s i n g ea c h m o d e . R e v i e w i n g

n u m e r o u s m o d e c h o ic e m o d e l s a p p l i e d i n t e c h n ic a l s tu d i e s , I h a v e o b s e r v e d t h a t a l l o f

t h o s e w h i c h h a v e fo r e c a s t s u b s t a n t i a l r a il t r a n s i t p a t r o n a g e h a v e b e e n p r e d i c a t e d u p o n

u n s u b s t a n t i a t e d a s s u m p t i o n s t h a t h i g h w a y t r a ff i c s p e e d s w i l l d e c l in e s ig n i f ic a n t ly ov e r

t w e n t y y e a r s , t h a t p a r k i n g f e e s w i ll r is e d r a m a t i c a l l y , a n d t h a t z o n i n g o r d i n a n c e s w i l l

b e c h a n g e d t o e l i m i n a t e th e p a r k i n g r e q u i r e m e n t s a s s o c i a t e d w i t h n e w b u i l d i n g s i n th e

t r a n s i t c o r r i d o r s .

M o d e c h o i ce m o d e l s w h ic h d i v i d e t r a v e l e rs a m o n g t h e a u t o a n d t r a n s i t m o d e s

i n c o r p o r a t e t h e e ff ec t o f a u t o m o b i l e o p e r a t i n g c o s t s a s w e l l a s p a r k i n g c o s t s a m o n g t h e

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72 MART1N WACHS

determinants of modal shares. Those favoring transit investments appear to select values

fo r automobile operating costs which yield inflated trans it patronage. For example, the

patronage forecast for the proposed Los Angeles subway derived automobile operating

costs from the assumption t ha t the average fuel efficiency of the region 's automobile f l e e t

would be 17.5 miles per gallon in the year 2000. Yet, the EPA is requiring automobile

manufacturers to achieve new car fleet averages of 27 miles per gallon by 1985, and that

agency states that the combined average for new and used car fleets is expected to be

24.6 miles per gallon by the year 2000. By using a figure of 17.5 miles per gallon, the

forecasters arrived at fuel costs which were 37 percent above the values which would

have been derived from an assumption of 24.6 miles per gallon. This, in turn led

to a systematic overestimate of the number of people who would switch from private

automobiles to the rail transit system (Trombly, 1984).

A similar device for inflating transit patronage forecasts is an assumption that all

competing bus service will be terminated upon completion of the rail transit system,

and tha t bus lines will be rerouted to become feeder services to the rail lines. While an

efficient, speedy bus route will in reality often compete with a rail line for passengers,

it cannot do so if it is assumed out of existence. And, many of the recent rail transit

patronage forecasts have assumed, some without making the assumption explicit, that

rail service will supplant rather than supplement parallel express bus service.

Indeed, it may seem obvious that efficient planning would result in the adoption of

specific policies to eliminate competing bus service in transit corridors, just as it might

be sound public policy to raise parking charges and reduce parking spaces provided in

corridors of rail transi t improvement. The only problem is that such actions are often

not actually taken when rail systems are eventually built. In the San Francisco Bay

area, for example, express bus service over the San Francisco-Bay Bridge continues to

be available at similar cost and faster door- to-door speed than the rail service provided

by BART (Webber, 1976). In Miami, no action has yet been taken to implement a four

hundred percent price increase assumed in the price of downtown parking when the

subway pat ronage forecasts were made (Davidson, 1984). In Washington, D.C., when

several bus lines were rerouted to serve as feeders to the new subway, many passengers

chose to drive to work rather than transfer from buses to the subway and back to buses,

despite the fact that the forecasting models assumed they would continue to make the

choice in favor of public transi t (Baker, 1983).

Many assumptions of model parameters and background conditions must be made,

and they are routinely adjusted to represent the rail transit system in such a way that it

will garner maximum patronage. If the patronage projected is insufficient to make the

case for funding the system, the parameters and assumptions are adjusted further. In

one technical report, I found that despite a series of assumptions which seemed favorableto a rail transit system, patronage forecasts in one application of the model appeared

too low to justify the cons truction of the system. The modelers merely reported tha t the

results appeared to be "unrealistic," and that the models were rerun with the highway

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F O R E C A S T S I N U R B A N T R A N S P O R T A T I O N P L A N N I N G : U S ES , M E T H O D S , A N D D I L E M M A S 7 3

speeds in 1990 lowered from 45 miles an hour to a "more realistic" 30 miles per hour

(Kaiser Engineers /DMJM, 1974). What was presented as a simple technical adjustment,

actually constituted an a s s u m e d increase of one third in the time required to make each

and every automobile trip in the entire metropolitan region. Naturally, this change gave

rise to a substantial increase in the forecast use of the proposed rail transit system.

Many additional examples could be presented to illustrate the ways in which the

complexity of the models distracts the user of the resulting forecasts from the fact that

the assumptions are far more critical to the outcomes than the mathematical character-

istics of the models. The systematic adjustment of assumptions is undertaken to prove

tha t patronage will justify the expenditure of public funds for subway construction.

After the systems are built and open to traffic, however, the assumed values of land

use, parking costs, auto travel speeds, and many other parameters of the model rarely

eventuate, and the patronage consequently rarely lives up to the forecasts.

7. Subw ay project cost forecasts

The forecasting of costs is in many ways quite a different sort of undertaking than the

forecasting of patronage. Cost forecasts are less dependent on social science models, and

more dependent on engineering judgments. Unit costs for tunneling, concrete work, la-

bor and equipment are estimated from recent projects of similar scope, and inflated to

reflect judgments about likely price trends over time. Like pat ronage estimation, how-

ever, cost forecasting is critically dependent on assumptions. While these assumptionsare often more explicit and more clearly stated than are many of the assumptions used

in patronage forecasting, they appear to be equally self-serving, and equally responsible

for the dramatic cost over-runs which characterize rapid transit projects. In the fol-

lowing paragraphs I will first discuss the ways in which assumptions contribute to the

errors associated with forecasts of capital (construction and rolling stock) costs; and

next consider their role in operating and maintenance cost forecasts.

7.1 Errors in forecasts of capi tal costs

Several studies have identified three major sources of errors in capital cost estimation

for rail transit systems. First, some projects have been changed during the design

phase, and changes in the scope of a major construction project often elevate its costs

substantially. For example, aft er construction had sta rted on the BART system, a law

suit was brought against it which sought to insure that the design of the system make

it accessible to passengers using wheelchairs. Eventually, the redesign of the system to

accommodate wheelchair patrons added many millions of dollars to its capital cost. The

second major source of error in capital cost forecasts has been the fact that assumed

rates of inflation have turned out to be much smaller than actual rates. Prices paid for

labor and materials have consequently been higher than the estimates. The third source

of error in capital cost estimation is delay. If construction takes place months or years

later than anticipated, because of labor disputes, lawsuits, delays in obtaining funding,

or other reasons, higher unit costs must be paid because of the continuing inflation of

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74 MARTIN WACHS

p r i c e s d u r i n g t h e t i m e w h i c h w a s lo s t . W h i l e t h e t h r e e f a c t o r s a r e d i s t i n c t i n s o m e

s e n s e s , t h e y a r e a l s o i n t e r r e l a t e d . C h a n g e s i n t h e s c o p e o f a p r o j e c t a r e o f t e n t h e c a u s e

o f m a j o r d e l a y s ; a n d w h e n i n f la t i o n i s a s s u m e d t o b e l o w e r t h a n t h e a c t u a l r a t e , c o s t

i n c r e a s e s d u e t o d e l a y s a r e e ve n g r e a t e r t h a n t h e y w o u l d h a v e b e e n h a d a m o r e r e a l i s t i c

r a t e o f i n f l at i o n b e e n a s s u m e d .

A n u m b e r o f s t u d i e s o f t r a n s i t c o s t e s t i m a t i o n h a v e c o n c l u d e d t h a t c h a n g e s o f

s c o p e , c o n s t r u c t i o n d e l a y s , a n d f a u l t y e s t i m a t e s o f i n f la t i o n r a t e s , e x p l a i n e d a s u b s t a n t i a l

p r o p o r t i o n o f t h e c o s t u n d e r e s t i m a t e s o f t h e s e s y s te m s : b e t w e e n f o r t y a n d n i n e t y p e r c e n t

o f t h e t o t a l . T h i s m e a n s t h a t a s u b s t a n t i a l p r o p o r t i o n o f t h e c o s t u n d e r e s t i m a t i o n

r e m a i n s u n e x p l a i n e d b y t h e s e f a c t o r s a n d i t i s u s e fu l , t h e r e fo r e , t o s e e k a d d i t i o n a l

r e a s o n s f o r t h e c o s t o v e r - r u n s . I n d o i n g s o , i t i s i n s t r u c t i v e t o c o m p a r e t r a n s i t p r o j e c t s

w i t h h i g h w a y p r o j e c t s w h i c h , a f t e r a l l, a r e a l s o s u b j e c t t o i n f l a t i o n , d e l a y s , a n d c h a n g e s

o f s c o p e , b u t w h i c h h a v e c o n s i s t e n t ly b e e n f o u n d t o b e c h a r a c t e r i z e d b y s m a l l e r co s t

o v e r r u n s t h a n r a i l t r a n s i t p r o j e c t s ( M e r e w i t z , 1 9 72 a n d 1 97 3; M e r e w i t z a n d S p a r k s ,

1 9 7 1 ) .

I n d i v i d u a l h i g h w a y p r o j e c t s a r e g e n e r a l l y s m a l l e r t h a n t r a n s i t p r o j e c t s , si n ce h i g h -

w a y n e t w o r k s c a n u s e f u ll y b e c o n s t r u c t e d i n sm a l l e r , in c r e m e n t a l b i t e s , a n d e a c h t w o

o r t h r e e m i l e s e g m e n t o f a h i g h w a y b e c o m e s a u s ef u l p a r t o f t h e o v e r a l l s y s t e m . T r a n s i t

p r o j e c t s , i n c o n t r a s t , a r e in h e r e n t l y m o r e " l u m p y " i n t h a t t e n o r fi f te e n m i l e s i s t y p i -

c a l ly th e m i n i m u m o p e r a t i o n a l s e g m e n t o f a s y s t e m . T h e s c a le o f e a c h t r a n s i t p r o j e c t

m a y w e l l m a g n i f y t h e e f f e ct o f a n y e r r o r s i n u n i t c o s t s . A n o t h e r s o u r c e o f t h e d i f f er e n c e

b e t w e e n h i g h w a y a n d t r a n s i t c o s t i n g er r o r s, h o w e v e r, m a y w e l l b e t h e t h e i n s t i t u t i o n a l

d i ff e re n c e s iia t h e f e d e r a l f u n d i n g p r o g r a m s w h i c h s u p p o r t c o n s t r u c t i o n p r o j e c t s i n t h e

t w o a r e a s .

F e d e r a l h i g h w a y f u n d i n g i s d i s t r i b u t e d t o t h e s t a t e s o n t h e b a s i s o f a f o r m u l a , a n d

i n d i v i d u a l h i g h w a y p r o j e c t s a r e u n d e r t a k e n i n o r d e r o f p r i o r i t y s e t w i t h i n t h e s t a t e s .

T h e p r o j e c t s c o n s t r u c t e d i n a g i ve n y e a r r e f le c t t h e i r r e l a t i v e p r io r i t i e s a n d t h e s iz e o f

t h e t o t a l h i g h w a y b u d g e t . T h e t o t a l b u d g e t i s r e l a t i v e ly f ix e d , a n d a c o s t o v e r r u n o n

o n e p r o j e c t r e q u i r e s t h e d e l a y o r c a n c e l l a t i o n o f l o w e r p r i o r i t y p r o j e c t s , b u t i t d o e s n o t

u s u a l l y a f f ec t t h e o v e r a l l f u n d i n g l e v e l s f o r h ig h w a y s . B e c a u s e t h e o v e r a l l f u n d i n g l e v e l

f o r h ig h w a y s i s r e l a t i v e l y i n d e p e n d e n t o f t h e c o s t o r a t t r a c t i v e n e s s o f a n y o n e p r o j e c t ,

t h e r e i s l i t t l e i n c e n ti v e t o d e l i b e r a t e l y d i s t o r t t h e c o s t s o f a n y o n e h i g h w a y p r o j e c t .

N e w r a i l t r a n s i t p r o j e c t s , u n l ik e h ig h w a y p r o j e c t s , a r e f u n d e d t h r o u g h a d is c r e -

t i o n a r y p r o g r a m . A d e c i s i on t o e x p e n d h u n d r e d s o f m i l l i o n s o f d o l l a r s i n on e y e a r

i n o n e c i t y d e p r i v e s o t h e r c i ti e s of t h e i r p r o j e c t s s i n c e t h e f u n d i n g is c o m p e t i t i v e a n d

n o t b a s e d o n a d i s t r i b u t i o n f o r m u l a . T h i s a d d s t o t r a n s i t f in a n c e p r o g r a m s a n i n c e n-

t iv e w h i c h d o e s n o t e x i s t i n th e c a s e o f h i g h w a y p r o j e c t s . L o w e r c o s t e s t i m a t e s p a y

o f f t o a g r e a t e r e x t e n t t h a n t h e y d o f o r h i g h w a y p r o j e c ts . A l o w c o s t e s t i m a t e m a k e s ap r o j e c t a p p e a r t o b e m o r e c o s t - e f fe c ti v e t h a n i t s c o m p e t i t o r s , w h i l e a m o r e c o n s e r v a t i v e

( h i g h e r ) c o s t e s t i m a t e m i g h t r e s u l t in t h e l o s s o f t h e e n t i r e p r o j e c t . O n c e a g a i n , a s w a s

t h e c a s e fo r p a t r o n a g e e s t i m a t e s , i t i s c l e a r t h a t t h e r e i s a n i n c e n t i v e f o r s el e c ti n g t h e

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F O R E C A S T S I N U R B A N T R A N S P O R T A T I O N P L A N N I N G : U S E S, M E T H O D S , A N D D I L E M M A S 7 5

most optimistic assumptions in development cost estimates for transit projects. And,

the absence of these incentives in highway cost estimation may help to explain the fact

that highway cost estimation generally turns out to be closer to the mark than does

transit cost estimation.

7.2 Errors in forecas ts o f t rans i t operat ing cos ts

As is the ease for capital costs, delays, inflation, and changes in project scope explain a

major proportion of overruns in operating cost in comparison with the initial forecast.

Still a substantial part of the error remains unexplained by these technical factors.

I reviewed the operating cost forecasts made for several transit proposals which have

been built , and several for which funding was not approved and which were consequently

not cons tructed. (Wachs and Ortner, 1979). All of the proposals incorporated sets of

assumptions which help to explain the departure of actual from forecast operating costs.

In all of these cases, for example, it was assumed that the ratio of transit employees

to vehicle-miles of service would be lower for the proposed systems than it was for any

presently operating trans it system. In other words, the operating cost forecasts assumed

a higher level of automation and a higher level of labor productivity than had actually

been achieved in any presently operating system to date. It has been asserted t hat each

new system will take advantage of the most modern technology, yet when each new

system actually comes on line it requires more labor than had been forecast. In part,

this has occurred because new technology has proved less reliable than anticipated.

While the favorable assumptions helped to gain acceptance of the subway proposals,

they were plainly unrealistic in light of operating experience with existing subways and

in light of a long history of transit labor.contract negotiations.

Operating costs are regularly estimated in transit forecasts as recurrent annual

costs assumed to grow over ten, fifteen, or twenty years of the project's life at a constant

annual rate equal to the general rate of inflation (Wachs and Ortner , 1979). In actuality,

however, during the pas t decade transit operating costs have been escalating at between

three and five percent per year above the general rate of inflation. This difference,

compounded annually for five to ten years, easily explains the discrepancy between

forecast and actual operating costs so common in rail transit. What it does not explain,

however, is why it has become standard practice to forecast a rate of operating cost

increase which is systematically below the actual recent rate. The answer, again, is

probably to be found in the role which these cost forecasts play in the competition for

funds. Why would one forecaster adopt a more realistic operating cost estimate than

another, when the consequences of a realistic assessment of operating costs would make

a favored project look less cost-effective than the competition?

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7 6 M A R T I N W A C H S

8 . I m p l i c a t i o n s f o r f o r e c a s t e r s

W h i l e m o s t o f t h e e v i d e n c e p re s e n t e d i n t h is p a p e r h a s b e e n c i r c u m s t a n t i a l , I a m c o n -

v i n c e d t h a t m o s t o f t h e f o r e c a s ts u s e d in t h e p l a n n i n g o f A m e r i c a ' s r a il t r a n s i t s y s t e m s

a r e s t a t e m e n t s o f a d v o c a c y r a t h e r t h a n u n b i a s e d e s t i m a t e s p r o d u c e d b y p o l i ti c a l ly n e u -

t r a l a p p l i e d s o c i a l s c i e n t is t s . I b e l ie v e t h a t s i m i l a r c a s e s t u d i e s c o u l d b e p r o d u c e d f r o m

o t h e r s e c t o r s i n w h i c h f o re c a s t in g r o u t i n e l y p l a y s a n i m p o r t a n t r o le i n p o li c y d e b a t es :

e n e r g y , e n v i r o n m e n t a l p l a n n i n g , a n d h o u s i n g , f o r e x a m p l e .

G o v e r n m e n t s w i t h l i m i t e d r e s o u r c e s t o a l l o c a t e , a n d c i t i z e n s w h o r e l y o n p u b l i c

s e r v i c e s a n d p a y t h e i r c o s t s , w o u l d s e e m o n t h e s u r f a c e t o a s s u m e t h a t f o r e c a s t s o f

f u t u r e n e e d a n d c o s t a re e x e c u t e d w i t h o b j e c t iv i t y. T h e c o m p l e x i t y o f p l u r al i st i c a n d

t e c h n o l o g i c a l s o c ie t ie s , h o w e v e r , p la c e s m a n y b u r d e n s o n t h o s e w h o p r e p a r e f o r e c a st s ,

w h i c h m a k e o b j e c t i v i t y d i ff i cu l t t o a t t a i n . P u b l i c re s o u r c e a l l o c a t i o n is c o m p e t i t i v ei n t h a t t h e d e c i si o n t o f u n d a p r o j e c t i n o n e j u r i s d i c t io n m a y d e p r i v e a n o t h e r o f a

s i m i l a r o p p o r t u n i t y . P o l i t i c a l in f l u e n ce , f in a n c i a l g a i n , j o b s , a n d p r e s t i g e a l l f lo w f r o m

" w i n n i n g " c o m p e t i t i o n s f o r p u b l i c p r o j e ct s . T e c h n i c al e x p e r t s a r e o f t e n e m p l o y e d b y

a g e n c i e s w h i c h a d v o c a t e p a r t i c u l a r s o l u t i o n s t o c e r t a i n p r o b l e m s : n u c l e a r v s . f o s si l f u e l

p l a n t s f o r p o w e r ; h i g h w a y s vs . r a p i d t r a n s i t f o r u r b a n t r a n s p o r t a t i o n a n d s o o n .

S u c h p r e s s u r e s a r e i n te n s i f ie d b y s e v e r a l c h a r a c t e r i s t i c s o f t e c h n i c a l f o r e c a s t s w h i c h

h a v e a l r e a d y b e e n m e n t i o n e d . F i r s t , i t i s i m p o s s i b l e t o l it e r a l ly v e r i f y t h e a c c u r a c y o f

a f o r e c a s t u n ti l t h e p r o j e c t is b u i lt a n d i n o p e r a t i o n . A n d a t t h a t p o i n t , t h e p u r p o s e o f

a n a d v o c a c y f o r e c a s t is a l re a d y a c c o m p l i s h e d e v e n i f a p o s t h o c e v a l u a t i o n s h o w s t h e

f o r e c a s t t o h a v e b e e n s e v e r e l y i n e r r o r . S e c o n d l y , b e c a u s e e v e r y f o r e c a s t i s s o c r i t i c a l l y

d e p e n d e n t o n c o r e a s s u m p t i o n s , i t is i n e v i ta b l e t h a t t h e f o r e c a s t e r is p r e s e n t e d w i t h

m a n y o p p o r t u n i t i e s a t w h i c h p a r t i c u l a r a s s u m p t i o n s c a n b e a d o p t e d w h i c h w i ll l e a d

t h e o u t c o m e s i n c e r t a i n d i r e c t i o n s . T h i r d l y , t h e a c t i v i t y o f f o r e c a s t i n g i s t e c h n i c a l l y

s o c o m p l e x t h a t i t n ee d r e v e a l t o m o s t w h o r e a d t h e r e s u l t s r e l a ti v e l y li t tl e a b o u t i ts

a s s u m p t i o n s a n d m a t h e m a t i c a l s t r u c t u r e s .

G i v e n t h e s e c o n d i t i o n s , i t i s i n d e e d d i f f ic u l t t o w i t h s t a n d p r e s s u r e s t o p r o d u c e s e lf -

s e r v i n g f o r e c a s t s w h i c h a r e c l o a k e d i n t h e g u i se o f t e c h n i c a l o b j e c t i v i t y . B y p o l i t e l y

a g r e e i n g t o s p e a k o f f o r e c a s t s a s o b je c t iv e , p l a n n e r s , e n g i n e e r s , a n d e c o n o m i s t s w h o

p r e p a r e t h e m c a n m a i n t a i n t h e i r s e lf - re s p e ct a n d p r o f e s s io n a l i d en t it y . S i m u l t a n e o u s l y ,

a d v o c a t e s o f p a r t i c u l a r p o s i t io n s g a i n s t r e n g t h f o r t h e i r a r g u m e n t s b y v i r t u e o f t h e

s u p p o s e d l y " u n b i a s e d " t e c h n i c a l a n a l y s e s w h i c h t h e y c a n c it e. A n d p o l i ti c i a n s w h o

f in a ll y m a k e r e s o u r c e a l l o c a t i o n s c a l m l y a c c e p t f o r e c a s t s w h i c h c o n f i r m t h e i r p a r t i c u l a r

p r e c o n c e p t i o n s w i t h f a r l es s c r i ti c a l r e v ie w t h a n t h o s e w h i c h d o n o t . A l l t h r e e s e t s

o f a c t o r s - - t e c h n i c a l f o r e c a s t i n g e x p e r t s , a d v o c a t e s f o r a p a r t i c u l a r p o i n t o f v i ew , a n d

p o l i t i c i a n s - - g a i n b y p r e t e n d i n g t h a t a f o r e c a s t is a n o b j e c t i v e sc i en ti fi c s t a t e m e n t , a n d

g a i n m o r e i f i t i s a l s o a n e f f e c ti v e s t a t e m e n t o f a d v o c a c y i n a s t r u g g l e f o r r e s o u r c e s .I n k e e p i n g w i t h t h e i l l u s io n o f t e c h n i c a l o b j e c t i v i t y , w h e n t h e p a s s a g e o f t i m e h a s

s h o w n t h e v a s t m a j o r i t y o f d e m a n d a n d c o s t f o r e c a s ts f o r p u b l i c s e r vi c e s t o h a v e b e e n i n-

a c c u r a t e , c r i ti c s g e n e r a ll y h a v e c o n t e n d e d t h a t " i m p e r f e c t t e c h n i q u es " a n d " i n a d e q u a t e

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F O R E C A S T S I N U R B A N T R A N S P O R T A T I O N P L A N N I N G : U S ES , M E T H O D S , A N D D I L E M M A S 7 7

d a t a " w e r e t h e s o u r c e s o f t h e p r o b l e m s . R a r e l y h a s i t b e e n a r g u e d t h a t f o r e c a s t s h av e

d e l i b e r a t e l y b e e n d e s i g n e d t o p l a c e c e r t a i n p r o j e c t s i n a f a v o r a b le l i g h t a n d o t h e r s a t

a d i s a d v a n t a g e . R a r e l y h a s i t b e e n a r g u e d t h a t t h e s t r u c t u r e o f g o v e r n m e n t a l d e c is i o n

m a k i n g m a k e s s u c h e t h i c a l l y t r o u b l e s o m e u s e s o f f o r e c a s t s in e v i t a b l e .

T h e s i t u a t i o n d e s c r i b e d is in d e e d a n e t h ic a l d i l e m m a b e c a u s e o f t h e a m b i g u i t y

a n d c o m p e t i n g a l l e g i a n c e s i n h e r e n t i n f o r e c a s t i n g . T h e f o r e c a s t e r , i n a ll l i k e l i h o o d , w a s

e d u c a t e d a c c o r d i n g t o a t r a d i t i o n o f p o s i t i v i s m h a v i n g a l l e g i a n c e t o a s e t o f m e t h o d s

a n d t e c h n i q u e s r a t h e r t h a n t o p a r t i c u l a r o u t c o m e s i n a p o l i c y d e b a t e . I t is n e c e s s a r y to

m a k e a s s u m p t i o n s s o t h a t t h e t e c h n iq u e s c a n p r o d u c e u s e f ul f o r e c a s ts , a n d r e a s o n a b l e

a s s u m p t i o n s a r e n o t n e c e s s a r il y a b e t r a y a l o f a c o m m i t m e n t t o te c h n i c a l o b j e c ti v i ty . T h e

a g e n c y f o r w h i c h t h e f o r e c a s t e r w o r k s , h o w e v er , h a s a c o m m i t m e n t t o c e r t a i n p r o g r a m s

o r s o l u t io n s a n d b e l i ev e s t h a t t h e y c a n b e s h o w n t o b e s u p e r i o r t o o t h e r s o n t h e b a s i so f r e a s o n a b l e c r i t e r ia .

I n a d d i t i o n t o c o m m i t m e n t t o a b o d y o f t o o l s a n d t e c h n i q u e s , t h e f o r e c a s te r m u s t

a l s o h a v e l o y a l t y o r r e s p o n s i b i l i t y t o t h e a g e n c y w h i c h h e o r s h e s e r v e s , e i t h e r a s e m -

p l o y e e o r c o n s u l t a n t . T h e e m p l o y e e w i s h e s t o a d v a n c e a n d w a n t s t o b e c o n s i d e r e d b o t h

c o m p e t e n t a n d c o o p e r a t i v e b y h i s o r h e r s u p e r io r s . T h e c o n s u l t a n t w i s h e s t o b e c on -

s i d e r e d f o r f u t u r e c o n t r a c t s . R e w a r d s f l ow f r o m e f f e c t iv e s e r v i c e a s a n a d v o c a t e f o r t h e

i n t e r e s t s c l e ar l y i d e n t if i e d b y th e o r g a n i z a t i o n . S h o u l d t h e f o r e c a s t b e m a d e o n t h e

b a s i s o f c o r e a s s u m p t i o n s w h i c h s e e m m o s t f a v o r a b l e t o t h e f u r t h e r a n c e o f t h e o r g a -

n i z a t i o n ' s g o a l s ? F o r e c a s t s o f te n r e q u i r e s o m a n y a s s u m p t i o n s t h a t t h e r e i s l e e w a y t o

a l lo w t h e f o r e c a s t e r t o s a t i s fy b o t h o r g a n i z a t i o n a l g o a l s a n d t e c h n i c a l c r i t e r ia . I n d e e d ,

i f h e o r s h e h a s b e c o m e a " t e a m p l a y e r " a n d h a s i n t e r n a l i z e d t h e g o a ls o f t h e a g e n c y ,

t h e r e m a y n o t e v e n a p p e a r t o b e a c o n f l ic t b e t w e e n t h e t w o l o y a l t i e s . I n c a s e s w h e r e

t h e f o r e c a s t e r i s a w a r e o f t h e c o n f l ic t , a n d w h e r e r e a s o n a b l e t e c h n i c a l j u d g m e n t m a y

d e l i v e r f o r e c a s t s w h i c h t h e a g e n c y w o u l d r a t h e r n o t h e a r a b o u t , t h e f o r e c a s t e r f a ce s th e

p r o b l e m o f c h o o s i n g b e t w e e n a d v o c a c y a n d o b j e c t iv i t y . T h e r e w a r d s f o r a d v o c a c y a r e

c l e a r , w h i l e e v e n t h e c r i t e r i a fo r j u d g i n g o b j e c t i v i t y a r e a m b i g u o u s .

I t is c r i t ic a l l y i m p o r t a n t t h a t p u b l i c a d m i n i s t r a t o r s a n d f o r e c a s t e r s r ec o g n i z e t h el i m i t s o f te c h n i c a l f o r ec a s t s . T h e r e a r e fe w e t h i c a l g u i d e p o s t s i n c l u d e d i n t h e e d u c a t i o n

o f p r o f e s s i o n a l s , t h e c a n o n s o f p r o f e s s i o n a l s o c i e ti e s , o r t h e p r o c e s s e s o f p u b l i c p o l i c y

m a k i n g t o s u g g e s t h o w s u c h c h oi c es s h o u l d b e m a d e . T h e c h o ic e s a r e p e r s o n a l a n d s o m e -

t i m e s t r o u b l e s o m e . F r e q u e n t l y , t h e o p t i o n s b o i l d o w n t o s e r v in g t h e a g e n c y o r l e a v in g

i t s e m p l o y . B e c a u s e t h e a g e n c y i t s el f , a n d t h e p o l i t i c a l p r o c e s s in w h i c h i t is e m b e d d e d ,

c o n t i n u e t o d e s c r i b e , r e s p o n d t o , a n d r e g a r d a d v o c a c y a s i f i t w e r e t e c h n i c a l l y o b je c t i v e

a n d n e u t r a l e x p e r t i s e , o n ly t h e m o s t s e n s it iv e o f a n a l y s t s w o u l d c h o o s e n o t t o s e r v e a s

a d v o c a t e . T h e r e s u l t i s t h a t m a n y f o r e c a s t s ar e s t a t e m e n t s o f h o p e a n d i n t e n t i o n , w h i l e

a n a l y s t s , a g e n c y b o a r d s , a n d p o l i t ic i a n s c o o p e r a t i v e l y m a i n t a i n t h e f ic t io n t h a t t h e y a r e

v a l u e - f r e e p r o j e c t i o n s o f t r e n d s . F e w f o r e c a s t e r s e n g a g e i n b l a t a n t f a l s i f i c a t io n i n o r d e r

t o r e c ei v e a c o m m i s s i o n o r p r o m o t i o n . M a n y , h o w e v er , a r e tr a n s f o r m e d i n s u b t l e s t e p s

f r o m a n a l y s t t o a d v o c a t e b y th e s i t u a t i o n in w h i c h t h e y p e r f o r m t h e i r w o r k . F u r t h e r

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r e sea r ch i s needed t o c l a r i f y t he ro l e s w h i ch fo r ecas t s p l ay i n po l i cy mak i ng , and t he

d i l emmas w hi ch po l i cy mak i ng ro l e s p r e sen t f o r f o r ecas t e r s .

R e f e r e n c e s

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F e b r u a r y .

W e b b e r , M e l v i n M : 1 9 7 6 , ' T h e B A R T E x p e r i e n c e - - W h a t H a v e W e L e a r n e d ' , M o n o -

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Telecommunications Forecasts Telecommunications Forecasts for the New Year

Experimental Forecasts of Seasonal Forecasts of Tropical

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MARYLAND DEPARTMENT OF TRANPORTATION STATE … · maryland department of tranportation . state highway administration . research report. integration of ramp metering and off-ramp

Asia Pacific Pay TV Forecasts Table of Contents Pacific Pay TV Forecasts... · • Executive summary and regional forecasts, ... • Regional forecasts summary from 2010 to 2023 by

Forecasts for the UK economy€¦ · A comparison of independent forecasts, June 2015 Average of new* forecasts Independent+ Averages June Forecasts for the UK economy June 2015 3

Development of Urban Infrastructure that Supports the Capital · ※ Figures for urban expressways are forecasts as of April 1, 2014. ※ Totals in each column may not match due to

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Information Acceleration: Validation and Lessons from the ...ebusiness.mit.edu/urban/papers/information... · compare actual sales to forecasts modified for the actual marketing plan

Introduction to Extraction,Tranportation and Loading Methods in Data Warehouses

Presentation of liner trade multimodel tranportation

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Tranportation and Transshipment Problem Ppt

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GMAO’s Seasonal-to-Interannual Forecasts & Potential contributions to MME Forecasts

MARYLAND DEPARTMENT OF TRANPORTATION RESEARCH … · maryland department of tranportation state highway administration research report advanced data analytics and mesoscopic models

Code of practice for the tranportation of wines, spirits ... · the tranportation of wines, spirits and concentrated grape must in ... Train and utilize a marine Surveyor. (5)

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Waikiki tranportation study

Annual Forecasts

Weather Forecasts

Housing Market Forecasts - Hamptons International Market Forecasts Autumn 2013

1. History of Tranportation from Prehistoric perido

Active Tranportation Planning - athenspublichealth.org Transportation Planning.pdf · planning? Active transportation plans (ATPs) are documents that provide a roadmap for improved

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APPENDIX I - AER I... · 2.3.1 Forecast SEQ Urban land values Urbis has forecast SEQ Urban land values from 2011/2012 to 2016/2017. These forecasts are presented in Chart 2.1. Historically,