E 330 Speed
The speed adopted by a driver depends on the physical characteristics of the highway and its roadsides; the weather; the presence of other vehicles; the speed limitations, either legal or control devices; and the attitude and judgment of the driver in evaluation these conditions. While any one of these may govern, often these conditions are combined.
Most drivers (about 85%) do not constantly adjust their speed to the street or highway, but rather choose some approximate uniform speed that seems to be safe for a given set of existing conditions. The other 15% will drive at either a higher or lower rate than the prevailing rate adopted by this majority.
It is impractical to design the street or highway to accommodate this 15%, since it is economically unfeasible to design to the higher standards required for a faster driver. On the other hand, designing for the slower driver may be more economical with respect to construction, but would probably result in higher vehicular operating costs, dissatisfaction to the majority of drivers, and danger to the faster drivers when adverse traffic conditions prevail.
On rural highways, the physical characteristics of the highway largely govern the speed of the driver when traffic and weather are favorable. State maximum speed laws and economic considerations, such as providing limited access highways, grade separations, etc., limit the designer in determining some of the physical characteristics of the street or highway. Therefore, under ideal conditions, most rural highways are designed for a maximum speed of 65 miles per hour.
On city streets, speeds are governed by the presence of other vehicles en masse, rather than by the physical characteristics of the street. Added factors are the control of speeds by legal authority and other retarding forces such as pedestrians, side friction from constant vehicular ingress and egress, turning movements, closely spaced intersections, and other physical and psychological conditions peculiar to urban environs.
In view of these factors, the City has adopted for City streets the tabulated form shown in Figure E 230, below.
Based on the above design speed for a given highway classification, other design criteria and limitations, such as maximum superelevation, minimum sight distances, horizontal curvature, etc., are also discussed in this Street Design Manual.