05_5_torbic_recommendedchangesinfuturedesignvehicles

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Recommended Changes in Future

Design Vehicles for Purposes ofGeometric Design of U.S. Highways

and Streets

Authors:

Darren J. TorbicDouglas W. Harwood

June 21, 2006

9th International Symposium on Heavy Vehicle

Weights and Dimensions


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Objectives

NCHRP Report 505: Review of TruckCharacteristics as Factors in Roadway Design

Ensure that geometric design criteria for trucks can

reasonably accommodate the dimensions of current and

future trucks using the U.S. highway system Recommend appropriate modifications and/or additions to

the AASHTO Green Book

Focus on Design Vehicle Fleet


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Outline

Overview of design vehicles Future changes to the U.S. truck fleet

Single-unit trucks

Single-trailer combinations Double-trailer combinations

Triple-trailer combinations

Summary of design vehicle recommendations


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Overview of Design Vehicles

Physical characteristics and proportions of vehiclesrepresent key controls in highway geometricdesign

2004 AASHTO Green Book presents design

vehicle dimensions and characteristics for 19design vehicles 8 trucks

Design vehicles:

Represent categories of vehicles Do not represent typical vehicles, but largest vehicles in

class

Selected or chosen by designer


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Future Changes to U.S. Truck Fleet

Charged with assessing effect on geometric design of bothcurrent and future truck populations

Current truck fleet assessed using 1997 Vehicle Inventory

and Use Survey (VIUS) and field data

Future truck population hypothesized based upon: Current truck population

Trends in current truck population

Likelihood of changes in size and weight laws and regulations

Economics of trucking industry

TRB Special Report 267 - Regulation of Weights, Lengths, and

Widths of Commercial Motor Vehicles (2002)

North American Free Trade Agreement (NAFTA)


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Recommendations


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Single-Unit Trucks

Current SU design vehicle

2 axles

Overall length 9.15 m (30 ft) Wheelbase 6.10 m (20 ft)

Recommend retaining current SU design vehicle and adding

a second larger SU design vehicle

Based on numbers and VMT


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Single-Unit Trucks (cont)

New recommended SU design vehicle [SU-8 (SU-25)]

3 axles

Overall length 12.04 m (39.5 ft)

Wheelbase 7.62 m (25 ft)


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Single-Trailer Combinations

Green Book includes 4 classes of single-trailer

combination design vehicles: WB-12 (WB-40) [10.1 m (33 ft) trailer]

WB-15 (WB-50) [13.0 m (42.5 ft) trailer]

WB-19 (WB-62) [14.6 m (48 ft) trailer]

WB-20 (WB-65 or WB-67) [16.2 m (53 ft) trailer]


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Single-Trailer: WB-12 (WB-40)

Rarely seen today on highways

May be appropriate for design of local roads and streets

Appropriate as container-carrying vehicle Retain as design vehicle but indicate:

Appropriate for local streets not used by larger combinationvehicles and for access roads to ports and train yards wherecontainer traffic might predominate


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Rarely seen today on highways

Prior to 1982 Surface Transportation Assistance Act (STAA), trailersize quite common

STAA mandated use of 14.6 m (48 ft) trailers on NN

Today 12.2 13.7 m (40 45 ft) trailers have largely disappeared 8 % of single-trailer combinations

Very limited use as design vehicle

Recommend eliminating as design vehicle in Green Book


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Nearly largest tractor-semitrailer combination on highway at one time WB-20 (WB-65 or WB-67) with 16.2 m (53 ft) trailer perhaps more common today

Retain because it represents:

Vehicle size limit specified by law Very closely the offtracking performance of longer trucks with rear axles pulled forward to

meet kingpin to center of rear tandem (KCRT) distance requirements

Slightly modify KCRT Current KCRT distance on WB-19 (WB-62) is 12.3 m (40.5 ft)

KCRT distance of 12.5 m (41 ft) more common (19 states)


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Can operate in most states on NN and state routes

VIUS data indicate 22.4 % of VMT by trucks are single-trailers

with 16.2 m (53 ft) trailers or more

Trucks with 16.2 m (53 ft) trailers constitute 47 % of all

combination trucks


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Identical to WB-20 (WB-65) except rear tandem is positioned closer to rear of trailer KCRT of WB-20 (WB-65) is 13.26 m (43.5 ft)

KCRT of WB-20 (WB-67) is 13.87 m (45.5 ft)

No good reason to include both vehicles in Green Book Recommend including only WB-20 (WB-67):

Has greater turning radius, offtracking, and swept path width

If designer is considering the offtracking and swept path of a single-trailercombination with a 16.2 m (53 ft) trailer and 12.5 m (41 ft) KCRT WB-19 (WB-62) should be used


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8 states permit trucks with trailer lengths > 16.2 m (53 ft) to

operate on NN

Not recommended for inclusion as design vehicle at present time Appropriate design vehicle developed for future consideration

should numbers and proportions increase

Designated WB-22 (WB-71)

Represents incremental increase in size to WB-20 (WB-67)


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Single-Trailer: 6 Axle Combination

Future legislation may allow states to issue permits

for 6 axle (i.e., single steering, tandem drive, andtridem rear axles) tractor-semitrailers withmaximum weights up to 35,400 kg (90,000 lb) Recommendation provided in TRB Special Report 267

6 axle tractor-semitrailers allowed to operate now No advantage over 5 axle combinations with current GVW

and axle limits

6 axle tractor-semitrailers would impact pavement

and bridge design, but would not impact geometricdesign Not recommended for inclusion in Green Book


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Double-Trailer Combinations

Green Book includes 2 classes of double-trailer

combination design vehicles: WB-20D (WB-67D) with twin 8.7 m (28.5 ft) trailers

WB-33 (WB-109D) with two 14.6 m (48 ft) trailers


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Double-Trailer: WB-20D (WB-67D)

Permitted to operate on NN since 1982

Common on intercity roads

Circumstances where WB-20D (WB-67D) would be theappropriate design vehicle are limited

Has less offtracking and swept path width than WB-19 (WB-62) andWB-20 (WB-65 and WB-67) which operate on similar roads

Retain because represents maximum vehicle size limit specified

in Federal law


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If current freeze on size and weight limits were lifted, a

combination with 10.1 m (33 ft) trailers could be legalized

TRB Special Report 267 recommends such a combination Not recommended for inclusion as design vehicle at present time

Appropriate design vehicle developed for future consideration should

such a truck become legal and common


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Also referred to as turnpike double

Consists of tractor with two 14.6 m (48 ft) trailers Operate under permit on specific roads

May be appropriate design vehicle on roads wherethey operate in substantial numbers

Current design vehicle uses a cab-over tractor Conventional tractor would be more realistic

Larger wheelbase of conventional tractor would haveminimal effect on offtracking

No recommended changes to current WB-33D (WB-109D) design vehicle


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If current freeze on size and weight limits were lifted, a

combination with 16.2 m (53 ft) trailers could be of interest

Economically advantageous to move low density commodities

Not recommended for inclusion as design vehicle at present time

Appropriate design vehicle developed for future consideration should

such a truck become legal and common


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Rocky Mountain doubles are fairly common in 20 states

Cross between the twin-trailer and turnpike double

May offtrack more than other relatively common truck types Recommended for inclusion in Green Book


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Triple-Trailer: WB-30T (WB-100T)

Consists of three 8.7 m (28.5 ft) trailers

One semitrailer and two full trailers

Represents most common triple-trailer combination on road

No recommended changes to this design vehicle


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Summary of Design Vehicle

Recommendations


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Single Unit Trucks

Retain current 2 axle SU

Designate as SU-6 (SU-20)

Add longer 3 axle SU

Designate as SU-8 (SU-25)


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Double-Trailer Combinations

Retain current WB-20D (WB-67D) with 8.7 m (28.5 ft) trailers

Configuration specified in Federal law WB-23D (WB-77D) with 10.1 m (33 ft) trailers developed for

future consideration

Retain current WB-33D (WB-109D) with 14.6 m (48 ft) trailers

(i.e., turnpike double) WB-37D (WB-120D) with 16.2 m (53 ft) trailers developed for

future consideration

Add WB-30D (WB-92D) with 14.6 m (48 ft) semitrailer and

8.7 m (28.5 ft) full trailer (i.e., Rocky Mountain double)

WB-23BD (WB-75BD) developed for future consideration

(i.e., B-train double)


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Triple-Trailer Combinations

Retain current WB-30T (WB-100T) with 8.7 m

(28.5 ft) trailers


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