Stopping Sight Distance. Stopping sight distance is the distance ahead that a motorist
should be able to see so that the vehicle can be brought safely to a stop short of an
obstruction or foreign object on the road. This distance will include the driver’s reaction
or perception distance and the distance traveled while the brakes are being applied.
The total distance traveled varies with the initial speed, the brake reaction time, and the
coefficient of friction for wet pavements and average tires. The values in Table 2.2 were
developed using a reaction time of 2.5 s and a braking deceleration rate of 11.2 ft/s2
(3.4 m/s2). The height of eye was taken as 3.50 ft (1.07 m) and the height of the object as
2.00 ft (0.61 m).
When considering the effect of stopping sight distance, it is necessary to check both
the horizontal and the vertical stopping sight distance. Horizontal sight distance may be
restricted on the inside of horizontal curves by objects such as bridge piers, buildings,
concrete barriers, guiderail, cut slopes, etc. Figure 2.6 shows a diagram describing how
horizontal sight distance is checked along an extended curve. Both formulas and a
nomograph are provided to enable a solution. Many times, where the curve is not long
enough or there are a series of roadway horizontal curves, a plotted-out “graphic” solution
will be required to determine the available horizontal sight distance.