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THE SPEED AND SAFETY EFFECTS OF REPLACING DIFFERENTIAL WITH UNIFORM SPEED LIMITS ON RURAL FREEWAYS
thesisposted on 28.04.2021, 20:43 by Raul A. Pineda Mendez
After the repeal of the National Maximum Speed Law in 1995, state governments were allowed to set speed limits on their interstate freeways. Several states adopted uniform speed limits (USLs) while others implemented differential speed limits (DSLs), namely lower speed limits for heavy vehicles and higher posted speeds for light vehicles. Indiana’s current speed limit law for rural freeways allows passenger cars to travel up to 70 mph and restricts large vehicles' speed with a gross weight of 26,000 pounds to 65 mph.
Previous studies comparing the safety performance of USLs and DSLs have yielded inconclusive results. This dissertation developed a new methodology to estimate the mobility and safety effects of changes in statewide speed limits and applied it in a case study for rural interstate freeways in Indiana. Unlike previous studies on speed limits, the effects of the speed limits were estimated hourly. Typically, a speed limit’s effects are assessed under low-density conditions close to free flow, but the proposed methodology calculates a speed limit’s effect on speed and safety under various congestion levels. Additionally, recognizing that there are differences in driving behavior, the speed limit's effect can be estimated separately for passenger cars and heavy trucks. The proposed hourly analysis in this methodology also considers previously omitted factors such as travel speed characteristics, weather conditions, lighting, and other seasonal variables.
Advanced statistical models were used in this dissertation to connect the speed limit with the operating travel speed, probability of crash, and probability of injury or death given crash occurrence. The effects of speed limits on travel speeds were estimated using multiple linear regression, while the effects on crash risk and severity were estimated using logistic regression with random parameters that accounted for unobserved variability. These models were then used in a statistical simulation to calculate the effects of alternative statutory speed limit scenarios, namely USLs. These scenarios were subsequently compared to the current speed limit policy in terms of their impacts on travel time, vehicle operating costs, and safety outcomes.
This dissertation found that speed limits affected mobility and safety, mostly under non-congested traffic conditions, but no statistically significant effects of the speed limits were found under congested traffic conditions. A marginal detrimental effect of DSLs on crash injury severity was detected under intermediate traffic conditions. These results suggest that replacing the current DSL, i.e., 70 mph for passenger cars and 65 mph for heavy trucks, on Indiana rural interstate freeways with a USL of 70 mph may yield benefits in terms of safety and mobility.