Highway Speed, Flow and Density
LOS Speed Range (mph) Flow Range (veh./hour/lane) Density Range (veh./mile)
A Over 60 Under 700 Under 12
B 57-60 700-1,100 20-Dec
C 54-57 1,100-1,550 20-30
D 46-54 1,550-1,850 30-42
E 30-46 1,850-2,000 42-67
F Under 30 Unstable 67-Maximum
Source: Homburger, Kell and Perkins, 1992.
Maximum Traffic Volumes (Passenger Cars Per Hour Per Lane)
4-lane Freeway 700 1,100 1,550 1,850 2,000
2-lane Highway 210 375 600 900 1,400
4-lane Highway 720 1,200 1,650 1,940 2,200
Highway Performance
The above two tables show the relationships between traffic speed, volume and density for a highway, and how these factors relate to Level of Service ratings. Traffic speed and flow on urban streets are determined primarily by intersection capacity, which is affected by traffic volumes on cross streets and left turn signal phases. As these tables indicate, traffic congestion is a non-linear function, meaning that a small reduction in urban-peak traffic volume can cause a proportionally larger reduction in delay. For example, a 5% reduction in traffic volumes on a congested highway (for example, from 2,000 to 1,900 vehicles per hour) may cause a 10-30% increase in average vehicle speeds (for example, increasing traffic speeds from 35 to 45 miles per hour). As a result, even relatively small changes in traffic volume or capacity on congested roads can provide relatively large reductions in traffic delay.