Temporal transferability assessment of injury severity models for single-vehicle and multi-vehicle crashes at highway ramp areas

Background: A ramp is an auxiliary roadway that facilitates the vehicles joining and leaving the main traffic stream of highway. Ramp areas are prone to road crashes because of the merging, diverging, and weaving traffic entering and leaving the highways. Objectives: This study evaluates the differences in injury severity and influencing factors between single- and multi-vehicle crashes at ramp areas, with which the transferability assessment of models across time periods is considered. Method: Separate injury severity models for single- and multi-vehicle crashes are established based on comprehensive crash data from North Carolina State in 2016-2018. Random parameter multinomial logit regression model with heterogeneity in means and variances is adopted to measure the association between crash injury severity and possible influencing factors, with which the effect of unobserved heterogeneity is accounted. In addition, partially constrained and temporal unconstrained modeling approaches are adopted to consider temporally shifting parameters. Results: Results indicate that there are considerable differences in the effects on injury severity between single- and multi-vehicle crashes, after controlling for unobserved heterogeneity and temporal instability. Some variables including aberrant driving, vehicle type, area type, speed limit and crash location are found to be significant only in one type of crash but not in the other. There are opposite effects for the crashes in rural areas on the likelihood of injury between singlevehicle and multi-vehicle crashes. Additionally, temporal transferability and out-of-sample prediction performance for models of single- and multi-vehicle crashes are assessed. Results indicate that remarkable temporal stability and instability coexist. Practical Applications: Findings should shed light on the effective traffic management and control strategies that can mitigate crash and injury risk at highway ramp areas.