Date of Award


Publication Type

Master Thesis

Degree Name



Civil and Environmental Engineering


Collision risk, Crash potential index, Differential variable speed limit, Heavy Trucks, Lane change conflicts, Spatial coordination


Chris Lee




This study develops an advanced speed control strategy called “differential variable speed limit (DVSL)”. DVSL assigns different speed limits for car and truck, and it also varies speed limits based on traffic conditions. The study tested the impacts of DVSL on delay and safety for a section of the Gardiner Expressway in Toronto, Canada. In this study area, recurrent congestion occurs due to a sudden increase in entry traffic volume from an on-ramp after it is open. Traffic in uncongested and congested periods was simulated using the VISSIM traffic simulation model. The impacts of the following four different speed limit strategies on traffic were evaluated: 1) uniform speed limit (USL), 2) differential speed limit for car and truck (DSL), 3) USL & DSL (U&D) – i.e., USL at low truck percentage and DSL at high truck percentage, and 4) DVSL. It was found that DVSL showed the lowest delays for both car and truck among the four speed limit strategies. DVSL also had the lowest number of truck lane changes but it had higher number of car lane changes than USL and U&D. This increase in the number of car lane changes rather created more spacing between vehicles in the right lane upstream of the on-ramp. More spacing allowed the vehicles from the on-ramp to merge with the mainline traffic more smoothly. Safety impacts of DVSL were also assessed based on the “Crash Potential Index (CPI)” which represents the likelihood of rear-end crash between the lead and following vehicles. The result shows that among the four speed limit strategies, DVSL had the lowest CPI for car-following-car, car-following-truck and truck-following-car conflicts. This study demonstrates that DVSL can better control car and truck speeds to reduce delay and improve safety of car-truck mixed traffic flow on freeways.