Safety and Traffic Implications of Differential Car and Truck Speed Controls for Two-Lane Highways

Differential car/truck speed limits have been used to enhance road safety and increase the fuel efficiency of trucks. For two-lane highways, the safety of these differential speed controls could be affected by possible changes in the pattern of car and truck interactions and their associated crash risks, especially as it pertains to overtaking. In this paper, a microscopic simulation model is presented that assesses the safety and traffic implications of differential car/truck speed limits for two-lane highway operations, with emphasis on the overtaking maneuver. Three speed control strategies are considered: uniform posted speed limits (USLs), differential posted car–truck speed limits (DSLs), and differential mandated truck speed limits (MSLs). Mandated limits involve the use of on-board truck limiters or governors with preset maximum thresholds. The results from several simulation tests suggest that DSLs and MSLs reduce the average travel speed (ATS) of the traffic stream. DSL and MSL controls were found to slightly increase head-on time-to-collision (TTC) and percentage time spent following (PTSF). The total number of overtakes was found to increase slightly for DSL and MSL controls compared to USLs. While the number of car–truck overtakes increased significantly for DSLs and MSLs, the number of car–car overtakes resulted in a significant decrease. It was concluded that three measures of ATS, TTC, and car–car overtaking positively impacted safety, while PTSF, car–truck, and total number of overtakes negatively impacted safety because of differential speed limit controls.


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  • Accession Number: 01606136
  • Record Type: Publication
  • Files: TRIS, ASCE
  • Created Date: Jul 1 2016 3:08PM