SIGHT DISTANCE

Spring 2015

Stopping Sight Distance

Sight Distance

Decision Sight Distance

Passing Sight Distance

Note: A driver’s ability to see ahead is of the utmost importance in the safe and efficient operation of a vehicle on a highway

Example: Sight Distance

Stopping Sight Distance

Definition: The available sight distance on a highway that allows a vehicle traveling near the design speed to stop before reaching a stationary object in its path

Brake Reaction Time (d1)

Braking Distance (d2)

Stopping Sight Distance

1 1.47d Vt=

Braking Reaction Time Distance:

Where,

d1 = BRT distance (ft)

V = Design Speed (mph)

t = Braking reaction time (s)

Note: Usually 2.5 seconds is used as the BRT. However, some drivers can take as long as 3.5 seconds. The absolute minimum should be 1.64 seconds.

Stopping Sight Distance

2

2 1.075V

da

=

Braking Distance:

Where,

d2 = Braking distance (ft)

V = Design Speed (mph)

a = deceleration rate (ft/s2)

Note:

Deceleration rate for design: 11.2 ft/s2

Deceleration rate in emergency: 14.8 ft/s2

Stopping Sight Distance

2

2

3032.2

Vd

aG

=æ ö÷ç ± ÷ç ÷çè ø

Braking Distance (with Grade):

Where,

d2 = Braking distance (ft)

V = Design Speed (mph)

a = deceleration rate (ft/s2)

G = Grade in percent (- for downgrade, + for upgrade)

Stopping Sight Distance

2

2

3032.2

Vd

aG

=æ ö÷ç ± ÷ç ÷çè ø

Example: Calculate the braking distance for a highway section characterized with a design speed of 60 mph and a 8% downgrade

( )

2

2

60 3600444.4

11.2 30 0.35 0.0830 0.0832.2

d ft= = =æ ö -÷ç - ÷ç ÷çè ø

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700 800

Distance (ft)

Sp

eed

(m

ph

)

Stopping Sight Distance: Design Values

Stopping Sight Distance: Design Values

Decision Sight Distance

Definition: The available sight distance on a highway needed to detect an unexpected or otherwise difficult-to-perceive information in a roadway environment

Decision Time (d1)

Braking Distance (d2)

Same as SSD

Decision Sight Distance

General Guidelines:

Stop on Rural Highway: 3.0 s

Stop on Urban Highway: 9.1 s

Speed/Direction Change on Rural Highway: 10.2 - 11.2 s

Speed/Direction Change on Urban Highway: 14.0 – 14.5 s

Other Areas:

Interchanges

Toll PlazaNote: In some instances, DSD may not be economically feasible. In these cases, the use of suitable traffic control devices should be provided

Decision Sight Distance: Design Values

0

10

20

30

40

50

60

70

0 200 400 600 800 1000 1200 1400

Distance (ft)

Sp

eed

(m

ph

)

T=2.5 s T=10.2 s

Passing Sight Distance

Sum of four distances:

d1 – Distance traversed during the perception and reaction time + acceleration to the point of encroachment

d2 – Distance traveled while the passing vehicle occupies the left lane

d3 – Distance between the passing vehicle at the end of its maneuver and the opposite vehicle

d4 – Distance traversed by an opposing vehicle for 2/3 of the time the passing vehicle occupies the left lane

Definition: The sight distance needed for allowing a faster vehicle to pass a slower vehicle on a two-lane highway

Passing Sight Distance

Initial Maneuver Distance

Occupying Left-Lane Distance

Clearance Distance Opposing Vehicle Distance

PASSING SIGHT DISTANCE• Important assumptions

– The overtaken vehicle travels at uniform speed

– The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section

– The passing vehicle accelerates during the maneuver and its average speed is 10 mph higher than the overtaken vehicle

– When the passing vehicle returns to its lane, there is suitable clearance length between it and the oncoming vehicle in the other lane

Passing Sight Distance: Design Values

PASSING SIGHT DISTANCE

10 30 50 70

Mph

PASSING SIGHT DISTANCE

Based on HSM – Multiple Passing Zones

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

30 35 40 45 50 55 60 65 70 75

Passign Speed (mph)

To

tal

Dis

tan

ce (

ft)

Passing Sight Distance

ti=3.6; t2=9.3

ti=4.5; t2=9.3

ti=4.5; t2=10.5

PASSING SIGHT DISTANCE

• Important design consideration– The range of speeds are affected by traffic

volume– Low volume – few vehicles need to be passed– High volume – few opportunities to pass– Should provide as many as passing sections as

possible– Should not be confused with other warrants

used for designing no passing zones

PASSING SIGHT DISTANCE

• Important design consideration (cont’d)– Overtaking a vehicle is easier for

downgrade segments– Need to provide longer passing sight

distance for upgrade– In mountainous terrain, it may be more

economical to provide for a “climbing lane”

– The number of passing opportunities greatly affects the capacity of your facility

Criteria for Measuring Sight Distance

Driver Eye Height

Passenger Car: 3.5 ft

Large Trucks: 5.9 to 7.9 ft

SSD Object

2.0 ft

PSD Object

3.5 ft

Object

Measuring Sight Distance

Passing Sight Distance

Previous Detailed Calculations

Initial Maneuver Distance (d1):

1 1.472i

i

atd t v m

æ ö÷ç= - + ÷ç ÷çè ø

Passing Sight Distance

Where,

ti = time of initial maneuver (s)

v = average speed of passing vehicle (mph)

a = average acceleration rate (ft/s2)

m = difference in speed between passing and passed vehicles (mph)

Note: Average time for ti is between 3.6 and 4.6 seconds

Average acceleration a is between 1.4 and 1.5 ft/s2

Distance for Occupying Left Lane (d2):

2 21.47d vt=

Passing Sight Distance

Where,

v = average speed of passing vehicle (mph)

t2 = time passing vehicle occupies the left lane (s)

Note: Average time for t2 is between 9.3 and 10.4 seconds

Clearance Distance (d3):

Passing Sight Distance

Usually varies between 100 to 300 ft

Opposite Vehicle Distance (d4):

Assume to be 2d2/3

Note: Opposite vehicle assumed to travel at the same speed as the passing vehicle

Passing Sight Distance: Design Values

Passing Sight Distance: Design Values