Investigating the Affecting Factors of Speed Dispersion for Suburban Arterial Highways in Nanjing, China

Suburban arterial highways are usually characterized by mixed traffic environments, which is a major contributor to traffic crashes. It has been known that speed dispersion as a surrogate safety measure has a strong correlation with safety. The objective of this study is to identify the influencing factors of speed dispersion for suburban arterial highways. Two definitions of speed dispersion are proposed for comparison: (1) an individual vehicle speed variation along a highway segment and (2) a vehicle speed variation at a cross-section. Vehicle speeds, traffic composition, and driving interference data were obtained from high-resolution videos from 20 segments of the G205 highway in Nanjing, China. An exploratory factor analysis was used to detect initial relationships between latent influencing factors and 13 candidate variables selected based on traffic condition, road condition, and driving behavior. A multivariable regression model was applied to identify the impacts of latent influencing factors on speed dispersion. The results from the two models showed substantial differences. The road condition factor was not significant in the cross-sectional speed dispersion model, but was interpretive in the segmented speed dispersion model. Driving interferences and illegal driving behaviors had a greater effect on the segmented speed dispersion. Consequently, segmented speed dispersion showed a better performance for the analysis of suburban arterial highways. On the other hand, traffic disturbance caused by driving interferences and illegal driving behaviors is the greatest contributor to high speed dispersion on suburban arterial highways, which may be mitigated by effective traffic management measures. It is expected that this work will help traffic managers better understand speed dispersion in mixed traffic environments and to develop effective safety improvement strategies.