Stopping sight distance is one of several types of
sight distance used in
road design
Highway engineering is an engineering discipline branching from civil engineering that involves the planning, design, construction, operation, and maintenance of roads, bridges, and tunnels to ensure safe and effective transportation of people and ...
. It is a near worst-case distance a vehicle driver needs to be able to see in order to have room to stop before colliding with something in the roadway, such as a pedestrian in a crosswalk, a stopped vehicle, or
road debris. Insufficient sight distance can adversely affect the
safety
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
or operations of a roadway or intersection.
Stopping sight distance is the distance traveled during the two phases of stopping a vehicle:
perception-reaction time (PRT), and maneuver time (MT).
[American Association of State Highway and Transportation Officials (1994) ''A Policy on Geometric Design of Highways and Streets'' (pp. 117–118)] Perception-reaction time is the time it takes for a road user to realize that a reaction is needed due to a road condition, decide what maneuver is appropriate (in this case, stopping the vehicle), and start the maneuver (taking the foot off the accelerator and depressing the brake pedal). Maneuver time is the time it takes to complete the maneuver (
decelerating and coming to a stop). The distance driven during perception-reaction time and maneuver time is the sight distance needed.
The design standards of the
(AASHTO) allow 1.5 seconds for perception time and 1.0 second for reaction time.
The values of stopping sight distance used in design represent a near worst-case situation. For design, a conservative distance is needed to allow a vehicle traveling at design speed to stop before reaching a stationary object in its path. A generous amount of time is given for the perception-reaction process, and a fairly low rate of deceleration is used. The design sight distance allows a below-average driver to stop in time to avoid a collision in most cases.
Driver perception/reaction distance is calculated by:
: ''d''
PRT = 0.278 Vt (metric)
: ''d''
PRT = 1.47 Vt (US customary)
Where:
: ''d''
PRT = driver perception-reaction distance, m (ft)
: ''V'' = design speed, km/h (mph)
: ''t'' = brake reaction time, in seconds
Based on the results of many studies, 2.5 seconds has been chosen for a perception-reaction time. This time will accommodate approximately 90 percent of all drivers when confronted with simple to moderately complex highway situations. Greater reaction time should be allowed in situations that are more complex.
Braking distance is calculated by:
: ''d''
MT = (metric)
: ''d''
MT> = (US customary)
where:
: ''d''
MT = braking distance, m (ft)
: ''V'' = design speed, km/h (mph)
: ''a'' = deceleration rate, m/s
2 (ft/s
2)
Actual braking distances are affected by the vehicle type and condition, the
incline of the road, the available
traction, and numerous other factors.
A deceleration rate of 3.4 m/s
2 (11.2 ft/s
2) is used to determine stopping sight distance.
Approximately 90 percent of all drivers decelerate at rates greater than that. These values are within most drivers' ability to stay within his or her lane and maintain steering control. Also, most wet pavement surfaces and most vehicle braking systems are capable of providing enough braking force to exceed this deceleration rate.
Stopping sight distance (SSD) is the sum of reaction distance and braking distance
: SSD = ''d''
PRT + ''d''
MT
: SSD = 0.278 Vt + (metric)
: SSD = 1.47 Vt + (US customary)
See also
*
Assured clear distance ahead
In legal terminology, the assured clear distance ahead (ACDA) is the distance ahead of any terrestrial locomotive device such as a land vehicle, typically an automobile, or watercraft, within which they should be able to bring the device to a ha ...
*
Braking distance
Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction b ...
*
Design speed
The design speed is a tool used to determine geometric features of a new road or street during road design. Contrary to the word's implication, the design speed of the road or street is not necessarily its vehicle speed limit or maximum safe s ...
*
Reaction time
Mental chronometry is the scientific study of processing speed or reaction time on cognitive tasks to infer the content, duration, and temporal sequencing of mental operations. Reaction time (RT; sometimes referred to as "response time") is meas ...
*
Road traffic safety
Road traffic safety refers to the methods and measures used to prevent road users from being killed or seriously injured. Typical road users include pedestrians, cyclists, motorists, vehicle passengers, horse riders, and passengers of on-ro ...
*
Traffic psychology
Traffic psychology is a discipline of psychology that studies the relationship between psychological processes and the behavior of road users. In general, traffic psychology aims to apply theoretical aspects of psychology in order to improve traff ...
References
{{reflist
Wikibooks: Stopping Sight Distance
Road transport