Predicting the Impact of Changing Speed Limits on Traffic Safety and Mobility on Indiana Freeways

Abstract

After repeal of the National Maximum Speed Limit Law, states were allowed to set individual speed limits on their interstate roads. Several states opted for a uniform speed limit while others implemented differential speed limits. The current speed limit on Indiana rural freeways limits speed of passenger cars to 70 mph and restricts to 65 mph speed of vehicles with a gross weight of 26,000 pounds or more. Indiana's speed limit on urban freeways is mostly 55 mph, but varies from 50 mph on certain downtown sections to 65 mph on some suburban sections. Previous studies comparing uniform and differential speed limit settings as to safety and mobility produced inconclusive or conflicting results. This study evaluates the safety and mobility effects of alternative speed limit scenarios on Indiana interstate freeways. Differences in travel time, vehicle operation, and traffic safety were used to compare the speed-limit scenarios. The effect of speed limit was evaluated in hourly periods. The traffic conditions in these periods were classified as uncongested, intermediate, and congested and the speed limit effects were analyzed in relation to these conditions. Rural and urban freeways were analyzed separately and distinct speed models were developed for cars and trucks. Safety was estimated by probability of crash and the conditional probability of crash injury severity. Speed limit was found to affect mobility and safety mostly in non-congested traffic conditions, while no significant effects were found in congested conditions. A limited effect was detected in intermediate traffic conditions on rural freeways. Results indicate that replacing the differential 70/65 mph speed limit on Indiana rural roads with the uniform speed limit of 70 mph may be beneficial for both safety and mobility. Increasing speed limits on urban interstates is confirmed to be beneficial for mobility but detrimental to safety.

Using Connected Truck Trajectory Data to Compare Speeds in States with and without Differential Truck Speeds

“…A 1992 study of speed and crash incident data on Interstates 64, 77 and 81 found that speed variances for all vehicles were significantly higher in Virginia with a DSL compared to the same routes operating under a USL in West Virginia [11]. A recent study evaluating the potential impacts of changing speed limits on Indiana's freeways using the National Performance Management Research Data Set (NPMRDS) of average travel times aggregated by nearly 2.5 mile long freeway segments observed that replacing the DSL for rural interstates with a USL of 70 mph may be beneficial for both mobility and safety, while increasing speed limits for urban interstates may aid mobility while detrimentally impacting safety [12]. Although these evaluations yield extremely valuable and impactful results, they are highly time consuming and labor intensive and may not provide trajectory level detail due to inherent aggregation.…”