Page:Highway Needs of the National Defense.pdf/123

Rh STOPPING DISTANCES DETERMINE MINIMUM SIGHT DISTANCE REQUIRED

Proper highway design requires that the length of highway ahead visible to the driver of a vehicle be sufficiently long at all times to enable a driver to bring his vehicle to a safe stop in advance of an object unexpectedly appearing on the road surface. In accomplishing such a stop the driver must perceive the object, react to his decision that a stop is necessary, and apply the brakes. Following these actions the vehicle requires a period of time to come to rest, the needed time depending on the speed of the car and the condition of the braking system. The distance that the car travels during these periods is termed the stopping distance. The distance within which all but the few vehicles with brakes in poor condition can stop when traveling at the maximum reasonable speed which the highway should be capable of accommodating is the needed sight distance.

The minimum sight distances in table 13 are sufficient for emergency stops where vehicle speeds do not exceed the design speeds as shown in the table. For safe operation sight distance at least as great as these minima must be provided at all points on a highway, and at intersections the driver who is about to enter the highway must. have a clear view in either direction, right and left, equal to the safe stopping distance for the design speed of the highway.

Provision of these adequate stopping sight distances places certain limitations on many of the geometric features of the highway, such as maximum curvature, rates of change in gradient at crests, width of cuts, clearances to lateral obstructions along the traveled way, and the combination of many of these features with one another.

RELATION OF TRAFFIC VOLUME, OPERATING SPEED, AND PASSING SIGHT DISTANCE TO HIGHWAY CAPACITY

The maximum possible capacity of a highway is attained when the lanes are completely filled with vehicles following each other at a minimum distance. Under this condition all vehicles will be moving at the same speed, which is established by the speed of the slowest driver on the road.

Effect of traffic volume

The high degree of restraint placed upon every motorist when the possible capacity has been reached, or even approached, results in