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GEOMETRIC DESIGN OF HIGHWAYS
Elements of Design - Stopping Sight Distance

GEOMETRIC DESIGN OF HIGHWAYS

Elements of Design - Stopping Sight Distance

Reference AASHTO - A Policy on the Geometric Design of Highways and Streets, 6th Ed

A highway alignment is composed of a variety of design elements which are combined to create a road facility of intended function that serves traffic safely and efficiently. There are several principal elements of design which includes sight distance, superelevation, horizontal and vertical alignments etc. Sight distance is the length of the roadway ahead that is visible to the driver. Adequate sight distance is critical to the safe and efficient operation of a vehicle on a high-way so that the drivers can control the operation of their vehicles to avoid striking an unexpected object in the traveled way. Also sufficient sight distance will enable drivers to use the opposing traffic lane for passing other vehicles without interfering with oncoming vehicles.

Stopping Sight Distance (SSD): Available sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary abstacle in its path i.e; the sight distance at every point along a roadway should be at least that needed for a below-average driver or vehicle to stop. Stopping sight distance is the sum of two distances: (1) Brake reaction distance - the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied, and (2) Braking distance - the distance needed to stop the vehicle from the instant brake application begins.
Brake Reaction Time: Brake reaction time is the interval from the instant that the driver recognizes the existence of an obstacle on the roadway ahead that necessitates braking until the instant that the driver actually applies the brakes. The recommended design brake reaction time is 2.5 sec which exceeds the 90th percentile of reaction time for all drivers.

Braking Distance:

The approximate braking distance of a vehicle on a level roadway traveling at the design speed of the roadway may be determined from the following equation:

  • Metric Units
    • d B = 0.039 V 2 a where:
    dB=braking distance, m
    V=design speed, km/h
    a=deceleration rate, m/s2
OR
  • US Customary Units
    • d B = 1.075 V 2 a where:
    dB=braking distance, ft
    V=design speed, mph
    a=deceleration rate, ft/s2
A deceleration rate of 3.4 m/s2 (11.2 ft/s2), a comfortable deceleration for most drivers, is the recommended deceleration threshold for determining stopping sight distance.

Braking Distance on Grades:

The stopping distances needed on upgrades are shorter than on level roadways and viceversa. When a highway is on a grade, equation for braking distance is modified to account for the grade (G), as given below:

  • Metric Units
    • d B = V 2 254 [ ( a 9.81 ) ± G ] where:
    dB=braking distance, m
    V=design speed, km/h
    a=deceleration rate, m/s2
    G=grade, rise/run, m/m
OR
  • US Customary Units
    • d B = V 2 30 [ ( a 32.2 ) ± G ] where:
    dB=braking distance, ft
    V=design speed, mph
    a=deceleration rate, ft/s2
    G=grade, rise/run, ft/ft

G is calculated as the rise in elevation divided by the distance of the run and the percent of grade divided by 100.

Stopping Sight Distance (SSD):

Stopping sight distance is the sum of the distance traversed during the brake reaction time and the distance to brake the vehicle to a stop . For various speeds and on level roadways, SSD can be computed using the following equation:

  • Metric Units
    • S S D = 0.278 V t + 0.039 V 2 a where:
    SSD=stopping sight distance, m
    t=brake reaction time, 2.5 sec
    V=design speed, km/h
    a=deceleration rate, m/s2
OR
  • US Customary Units
    • S S D = 1.47 V t + 1.075 V 2 a where:
    SSD=stopping sight distance, ft
    t=brake reaction time, 2.5 sec
    V=design speed, mph
    a=deceleration rate, ft/s2

Stopping sight distances exceeding those calculated using equation above should be used as the basis for design.

Stopping Sight Distance (SSD)




Recommended design brake reaction time is 2.5 sec.

Recommended deceleration rate is 3.4 m/s2 (11.2 ft/s22).

For level roadways, please enter a G value of 0. For gradients, please enter negative G values for downgrades and positive values for upgrades.


Stopping Sight Distance (SSD): Results

Braking Distance (dB):

Brake Reaction Distance:

Calculated Stopping Sight Distance:

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