SIGHT DISTANCE

Spring 2016

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 1.075Vda

=

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

Vda G

= 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

Vda G

= 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

260 3600 444.4

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)

Spee

d (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 sStop on Urban Highway: 9.1 sSpeed/Direction Change on Rural Highway: 10.2 - 11.2 sSpeed/Direction Change on Urban Highway: 14.0 – 14.5 sOther Areas:InterchangesToll 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)

Spee

d (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 encroachmentd2 – Distance traveled while the passing vehicle occupies the left laned3 – Distance between the passing vehicle at the end of its maneuver and the opposite vehicled4 – 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 70Mph

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)

Tota

l Dis

tanc

e (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 ftLarge 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

iatd 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 secondsAverage 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 ftOpposite Vehicle Distance (d4):

Assume to be 2d2/3Note: Opposite vehicle assumed to travel at the same speed as the passing vehicle

Passing Sight Distance: Design Values

Passing Sight Distance: Design Values