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Safety Evaluation of Speed Limit Increase on Rural Highways in British Columbia

Author(s): Sengupta, Elesawey, Timpa, Jain

Slidedeck Presentation Only:

3B_Sengupta

Abstract:

Background/Context: In 2014, the British Columbia Ministry of Transportation and Infrastructure (MoTI) increased speed limits on 1,300 km of provincial rural highways. This decision was made based on over 300 speed surveys with measurement of the 85th percentile operating speed, which were upwards of 10 km/h higher than corresponding posted speed limits and also significantly downward trend of serious collisions since 2003. Since the increase, the Ministry committed to assessing the safety effect of speed limit increase following three years of implementation.

Aims/Objectives: As speed plays an important role in road safety, and traffic operations is enhanced when appropriate speed limits are set, the main objective of this analysis was to estimate the safety effects of the changed speed limits on rural highways after the three years of implementation.

Methods/Targets: Many factors such as the 85th percentile speed, collision rates and contributing factors were measured and assessed. However, the study design used to estimate the safety effects of the changed speed limits is a time-series analysis, which is often referred to as a before-after (BA) analysis. This approach measures the change in safety over time due to the implementation of a safety initiative. For BA analysis, Bayesian methods are commonly used within an odds-ratio (OR) analysis for their ability to: a) ensure that a noted change in the safety performance is caused by the safety initiative and not by other confounding factors or causes external to the initiative, and, b) treat unknown parameters such as predicted collision frequency as random variables having their own probability distributions.

Results/Activities: A statistically significant increase of 11..2% in the number of serious collisions was found following speed limit increases for all the affected segments.

Discussion/Deliverables: Increasing speed limits on a number of rural highway segments in BC resulted in an increase of serious collisions. Based on this study, MoTI rolled back speed by 10 km/h on all highway segments where serious collisions increased.

Conclusions: The Empirical Bayes evaluation showed statistically significant increases in serious collisions of 11.2.% on all speed limit increase segments. The results are consistent with the preliminary safety evaluation study done by UBC where the study found an increase of 11.1% in fatal and injury collisions after increasing speed limits.