Vehicle Speed Characteristics in States with Uniform and Differential Speed Limit Policies

The purpose of this study was to examine the travel speed characteristics of passenger vehicles, trucks, and buses in states with different speed limit policies, primarily to ascertain the effects of differential speed limits (DSLs) versus those of uniform speed limits (USLs) for large vehicles. Spot-speed studies were conducted in the neighboring states of Indiana, Michigan, and Ohio; these studies permitted examination of speed characteristics under several speed limit settings along the same freeways. The sites included urban and rural locations, with speed limits at individual locations that varied from 55 to 70 mph. These sites included USLs and DSLs, with 5 mph differentials on rural freeways in Indiana and 10 mph differentials at rural locations in Michigan. Spot-speed data were collected at 157 freeway sites in the three states along flat, tangent segments. Regression models were estimated to ascertain differences in the mean speeds, the 85th percentile speeds, and the standard deviation in speeds across locations. The results showed passenger vehicle speeds to be quite consistent across the three states where a common 70 mph limit was in effect. Speeds varied more at locations with lower posted limits and between trucks and buses. Speeds were most consistent in Ohio at locations with higher USLs. The variability in travel speeds for all vehicles was found to be highest on freeways with DSLs, followed by urban freeways with USLs of 55 mph.