The general factors which control the maximum rates of superelevation for highways also apply to intersection curves. See Section E 411, Superelevation. The desirable maximum for City streets is 0.06 foot per foot.
Due to right of way costs and other existing criteria, particularly on City streets within the intersection areas, the alignment limitations restrict the length of curves and radii that can be used. The amount of superelevation should be considered individually for each intersection. Indications of some conflicting situations are discussed below.
The driver will tend to slow the vehicle at intersections, particularly where other traffic is present. However, it is not a good practice to reduce the superelevation in anticipation of this, because frequently when no traffic is present, the driver may tend to continue at a normal rate. It is desirable to provide as much superelevation as practicable on intersection curves, especially where the intersection curve is sharp and on the downgrade. However, in considering turning movements around a corner, it must be recognized that the curb return radii are relatively short in comparison to the curve radii normally used at other than intersections. In addition, superelevation which is designed for turning around a corner in one direction may provide adverse superelevation when entering from another direction. Therefore, the amount of superelevation given for a turning movement should also include the other probable turning movements before a superelevation value is assigned.
Another factor that affects the amount of superelevation to be used is drainage control. Where horizontal curves require superelevation at an intersection, and where this intersection is at or near the summit of a vertical curve, there is usually insufficient water in the street to create a drainage problem. Where this same compensating factor is applied to a sag vertical curve or to a portion of a street carrying large amounts of water, even a small amount of superelevation may direct an appreciable amount of water either to the low side or down a street with a low water-carrying capacity. This may cause property damage or may necessitate an expensive drainage system.
Comments