Empirical Analysis of Truck and Automobile Speeds on Rural Interstates: Impact of Posted Speed Limits

Posted speed limit settings on rural highways have always been a point of contention with different stakeholders having very different perspectives (motorist, enforcement, commercial trucking, etc.). In particular, the effect of the posted speed limit on safety has been widely studied, primarily using accident data bases. The results reported in the literature are often inconclusive or even contradictory. In addition, many speed-related safety and environmental objectives are in conflict with mobility goals. An important aspect of this research relates to the impact of posted speed limits on actual traffic behavior. This study investigated the speed distributions for both heavy trucks and light vehicles (cars) at 19 rural interstate highway sites across the United States. The speed limit configurations were selected to encompass the full range of posted limits (55 mph to 75 mph) and to include both uniform and differential speed limits (e.g., 55 for trucks and 70 for cars). The results of the study describe the actual distribution of speeds for trucks and cars across the various speed limit configurations. In addition, the mean speeds, 85th percentile speeds, compliance rates and observed speed differentials are reported for the individual sites and for each speed limit configuration. The final set of data demonstrates the effect of increased fuel costs on the distribution of truck and car speeds. The results of the study provide an important contribution to the discussion of appropriate maximum speed limits, as well as the natural differential speeds that exist between heavy trucks and light vehicles.