Stopping sight distance
Stopping sight distance is the distance along a roadway required for a driver to perceive and react to an object in the roadway and to brake to a complete stop before reaching that object. Stopping sight distance should be provided at every point within a roundabout and on each entering and exiting approach.
National Cooperative Highway Research Program (NCHRP) Report 400, Determination of Stopping Sight Distances (8), recommends the formula given in Equation 6-2 for determining stopping sight distance (presented in metric units, followed by a conversion of the equation to U.S. customary units).
Exhibit 6-28 gives recommended stopping sight distances for design, as computed from the above equations.
Stopping sight distance should be measured using an assumed height of driver’s eye of 1,080 mm (3.54 ft) and an assumed height of object of 600 mm (1.97 ft) in accordance with the recommendations to be adopted in the next AASHTO “Green Book” (8).
At roundabouts, three critical types of locations should be checked at a minimum:
- Approach sight distance (Exhibit 6-29);
- Sight distance on circulatory roadway (Exhibit 6-30); and
- Sight distance to crosswalk on exit (Exhibit 6-31).
Forward sight distance at entry can also be checked; however, this will typically be satisfied by providing adequate stopping sight distance on the circulatory roadway itself.
