Subarea-based modeling of crash severity in freeway ramp zones: Exploring heterogeneity and transferability

The frequent acceleration, deceleration and lane change of vehicles in the freeway ramp area lead to significant differences in the accident mechanism of the diverging, merging, and weaving zones. This study aims to analyze the heterogeneity and transferability of factors affecting the severity of accident injuries in different ramp sub-areas, and to make a comparative analysis of the significant influencing factors in different sub-areas.

Based on the accident data of freeway ramps in the state of Florida, the United States, this study divides the severity of accident injuries into three categories. Random parameter logit models with heterogeneity in both means and variances are developed for the overall ramp area as well as for diverging, merging, and weaving subareas. Choose 22 explanatory variables from the four aspects of driver, vehicle, roadway, and environmental characteristics. The log- likelihood ratio tests are used to evaluate the transferability of different ramp areas model parameters, and the heterogeneity of influencing factors is analyzed through the distribution characteristics of random parameters. At the same time, the direction and degree of influence of significant variables on the severity of accident injury are quantified by using the average elastic coefficient.

The results show that there is no transferability of parameters between the overall model and subarea models or among the three ramp subareas, which verifies the necessity of zoning modeling analysis. Different ramp subareas show significantly different means and variances characteristics. "Hit and run: No," "Pre-crash speed: [20, 40)," and "Driver gender: Male" emerge as random parameters in diverging, merging, and weaving areas. Comparative analyses further reveal that some variables exert opposite effects on crash injury severity across different ramp zones, whereas others are significant only within a single subarea. Only a limited number of factors-such as driver condition, driver gender, low pre-crash speed, and vehicle type-consistently influence crash injury severity across all ramp subareas.

The factors affecting the severity of accident injuries in the freeway ramp area have significant spatial heterogeneity. The research results systematically analyzed the significant influencing factors of each subarea, and found that driver behavior, pre-collision speed, road speed limit, lighting conditions all have different effects in different subareas. Regionally differentiated improvement measures and management policies should be formulated for different subareas.