Stopping sight distance

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Stopping sight distance is a term used in highway design. It is defined as "the length of roadway ahead visible to the driver."^[1]

Contents 1 Sight distance 1.1 Stopping sight distance 1.2 Decision sight distance 2 Notes and references

[edit] Sight distance

Sight distance, in the context of road design, allows a vehicle travelling at design speed to be able to stop before reaching a stationary object in its path. Typically the design sight distance allows a below-average driver to stop in time to avoid a collision. ^[1]

[edit] Stopping sight distance

Stopping sight distance is the sum of the break reaction distance (the distance travelled between the time the driver sees an obstruction to when the brakes are applied) and the braking distance (the distance travelled while braking the vehicle to a stop).

The equation y=.6x²+1.1x can be used to find this, where x is equal to miles per hour and y is the final Stopping Sight Distance. (The original equation listed here was incorrect)

Miles Per Hour	Stopping Distance (in ft)
0	0
5	7
10	17
15	30
20	46
25	65
30	87
35	112
40	140
45	171
50	205
55	242
60	282
65	325
70	371
75	420
80	472
85	527
90	585
95	646

[edit] Decision sight distance

The decision sight distance includes an additional distance -- the distance travelled while deciding that what is seen is a hazard and deciding what to do. The decision site distance is "distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required maneuver safely and efficiently." ^[1] Ideally roads are designed for the decision sight distance -- typical times detecting and understanding a situation is about 6 to 10 seconds and 4 to 5 seconds to perform the right maneuver.

[edit] Notes and references