Effect of increasing the freeway posted speed limit on entry ramp speed-change lane crash frequency

Abstract

The objective of this research is to model the effect of increasing the freeway posted speed limit on entry ramp speed-change lane crash frequency. The entry ramps along I-85, I-77, I-277, and I-485 in Charlotte, North Carolina, United States were considered to collect data, identify critical predictor variables, develop safety performance functions (SPFs) by severity, and validate the SPFs. Data for 76 and 20 entry ramp speed-change lanes (reference and comparison groups) were used for modeling and validation/calibration, respectively. The Negative Binomial log-link distribution-based models were the best-fit as the crash data are over-dispersed. The traffic volume, the speed difference between the freeway and the ramp, and the ramp configuration are significant predictor variables to estimate total, property damage only (PDO), and non-incapacitating and possible injury crash frequency at entry ramp speed-change lanes. The Empirical Bayes (EB) method was then applied to examine the effect of increasing the freeway posted speed limit on 48 entry ramp speed-change lanes (treatment/test group) on I-485. The findings from the EB method-based analysis indicate that there was a decrease in the total, non-incapacitating and possible injury, and PDO crash frequencies at 18, 37, and 18 (38%, 77%, and 38%) of the selected 48 entry ramp speed-change lanes after the increase in the freeway posted speed limit on I-485. An increase or no change in the total, non-incapacitating and possible injury, and PDO crash frequencies was also observed at some entry ramp speed-change lanes after the increase in the freeway posted speed limit on I-485.