Page:Highway Needs of the National Defense.pdf/122

102 VARIATIONS OF TRAFFIC VOLUME AND CRITICAL HOURLY VOLUME DETERMINE REQUIRED HIGHWAY CAPACITY

Certain general facts pertaining to the volume and pattern of traffic movement are widely known. The daily pattern is one with which everyone is familiar; and the morning and evening rush hours, with intervening lulls, are accepted facts of the traffic day. Also, the fact that traffic on certain days of the year is much heavier than that on other days is a matter of common knowledge. The manner in which these variations in flow are related to the traffic-carrying ability of a roadway facility is not so readily apparent.

Investigations of the fluctuations in traffic movement show that a highway planned to accommodate the traffic demand during the peak hour of the average day of a year will be congested, on an average, during 160 days of the year. Also, if the road is planned to take care of the traffic during the average hour of any day the demand during the peak hour of that day will be more than double the capacity of the highway. It is apparent then, that the roadway should be designed to accommodate the traffic that moves during most of the heavier hours of the year, but that it is uneconomical to plan for the infrequent extremes that occur from time to time throughout the year.

Comprehensive studies have shown that it is uneconomical to design the average highway for a greater hourly volume than that which is exceeded during only 30 hours each year, and little will be saved in the construction cost and a great deal lost in expediting the movement of traffic if the highway is designed to accommodate fewer vehicles than the volume exceeded during the 50 highest hours of the year.

For the interstate highway system the thirtieth highest hour of the year has been selected as the criterion for decision upon the adequacy of existing facilities and for determinations of geometric features of the highways, such as the number of traffic lanes, maximum gradients, and curvatures, that are required to accommodate traffic using this system.

As a result of the variation in traffic flow, highways in certain sections of the country require considerably higher geometric standards than others, for the same total annual traffic volumes, in order to provide the type of service which this system of highways is expected to render.