TRAFFIC SPEED REPORT NO. 115

The attached Progress Report No. 115 is the report of the July-September 1981 study of free flow automobile and truck speeds on tangent, level and dry sections of rural interstate, 4-lane and 2-lane highways and on urban interstate highways in Indiana. The overall results show an average free flow speed of 56.9 mph, up only 0.3 mph from 1980. In fact, speeds have not changed much since the implementation of the 55 mph speed limit in 1974. This report includes a chart, Figure 2 on page 8, that graphically summarizes the results of summer free flow speed monitoring since 1970. Such charts have not been included in the Traffic Speed Reports since the summer study was combined with the study for compliance with the National Maximum Speed Limit in 1975. This chart shows that speeds of all classes of vehicles on all types of highways have not changed much since 1974. Generally speaking, the 55 mph speed limit caused the 85th percentile passenger car speeds to drop by about 7 mph, while the 15th percentile speeds remained unchanged. (Author)