Stopping Sight Distance
“Sight
Distance” is the length of roadway that a driver can see ahead. “Stopping
sight distance” (SSD) is the minimum sight distance required for a driver to
stop a vehicle on wet pavement after seeing an object without hitting the
object. SSD determines minimum lengths of vertical curves and minimum radii of
horizontal curves.
The American Association of State Highway and Transportation Officials (AASHTO) establish the minimum SSD. The SSD in Table 1 are for passenger cars and do not consider the distance necessary for trucks to safely stop. Generally, separate stopping sight distances for passenger cars and trucks are not considered. The extra sight distance provided by the higher seat position of a truck driver compensates for the extra distance needed to safely stop.
To
measure sight distance on an existing road, one person stands in the center of
the travel lane sighting from the top of a sighting rod, while another holds a
target rod at a length away in the direction of travel and records the distance
at the point in which the bottom two feet of the target rod moves out of view of
the person with the sighting rod. They compare this distance with the minimum
required stopping sight distance in Table 1.
The easiest way to prevent sight distance problems is to avoid them by ensuring new roads adhere to the minimum standards for stopping sight distance. Sight distance improvements may be costly but necessary in places where inadequate sight distance has resulted in crashes. Agencies should consider improvements when the recorded distance is less than the minimum requirement. It is most cost effective to combine sight distance improvements with other road improvements to make more cost effective. Traffic control devices, creating no passing zones, and establishing public awareness campaigns are techniques to improve the safety at locations where there is poor stopping sight distance.
References:
Mearkle, Jim, Nuggets & Nibbles, Cornell Local Roads Program, Vol.
XXII, No. 4
Geometric Design of Highways and Streets,
American Association of State Highway and Transportation Officials,
Washington, DC 2004
“Stopping Sight Distance,” Bay State Roads Program, Tech Note #25, 2004
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