Estimation of Travel Time Reliability for Freeways Using Mean and Standard Deviation of Travel Time

Variable speed limit (VSL) signs are in use across Interstate 255/270 (I-270/I-255) in St. Louis County, Missouri, varying the speed limit in real time during peak periods to reduce congestion. This paper evaluates the effects of the VSL system on travel time distribution and travel time reliability. Several distributions, including gamma, largest extreme value, log-logistic, log-normal, and Weibull, were fitted to the 24-hour travel time data to determine which provides the best fit for post-VSL conditions. Log-normal distribution provided the best. This paper also proposes a new method to estimate the 95th percentile of travel time, one of the most important factors for travel time reliability, using the mean and standard deviation of travel time. Parametric and nonparametric statistical tests were performed to validate this method using 36 field data sets from four congested segments of I-270/I-255.