driving conditions that are wholly intolerable. Drivers are entitled to exercise some freedom in their selection of speed and their ability to maneuver. The extent to which this liberty may be exercised is dependent primarily upon the volume of traffic using the highway and also, for two-lane roads, upon the portion of the highway where sight distances are adequate for overtaking and passing. Other elements such as narrow lanes and narrow shoulders also provoke congestion, but on highways built to the adopted standards for the interstate highway system these inferior features are not present.
Practical working capacity
The maximum volume of traffic that will permit a reasonable degree of freedom from congestion may be termed the practical working capacity of the facility. Its magnitude depends in large measure upon local conditions. For rural areas operating conditions are satisfactory so long as drivers who so desire may travel at average speeds of 45 to 50 miles per hour without undue hazard. On roads having a lane width of 12 feet and excellent alinement, such speeds can be attained with the traffic volumes shown below, for highways of different numbers of lanes:
- Two-lane highways: a total of 800 vehicles per hour.
- Three-lane highways: a total of 1,400 vehicles per hour.
- Four- or more-lane highways: 900 vehicles per hour per lane for the lanes in the heavier direction of travel.
These volumes include a normal percentage of trucks. The average speed of all traffic under these volume conditions will be about 40 miles per hour. Where a lower average speed is satisfactory, as is the case on multilane expressway facilities in urban areas where flow is uninterrupted, a volume of 1,350 mixed vehicles per lane per hour is practicable. The average speed at this volume will be about 30 miles per hour but drivers having a desire to do so would be able to average 35 to 40 miles per hour in safety.
Passing sight distance
Sight distances that restrict passing maneuvers are a serious detriment to the capacity of two- and three-lane roads. Where passing sight distances are inadequate, drivers are restricted in their freedom of movement in much the same manner as when the lane used for passing is filled with oncoming vehicles. The reduced capacity resulting from short sight distances can be determined by using as a criterion the percentage of the total highway on which sight distances are insufficient to permit passing maneuvers to be performed safely.
Studies have shown that few vehicles are passed when their speed is above 50 miles per hour, and that the majority of cars are moving slower than 45 miles per hour when they are passed. Sight distances within the range of 1,500 to 2,000 feet are those most widely needed to meet the requirements for passing vehicles traveling below 50 miles per hour. ‘The distance required to complete a passing maneuver is slightly longer if the vehicle passed is a commercial vehicle than if it is a passenger car, if the speeds of each are the same. Commercial vehicles, however, usually travel at lower speeds than passenger cars. Hence, the sight distance lengths quoted above are usually ample on roads where there are trucks and truck-trailer combinations.
