Modeling the endogeneity of lane-mean speeds and lane-speed deviations: a structural equations approach

This paper attempts to macroscopically address endogeneity issues related to lane-mean traffic speeds and lane-speed deviations. Methodologically, we seek to provide a better understanding of mean speeds and speed deviations across the lanes of a multilane highway. In so doing, the work may eventually be applied to better understand highway safety and the effects that lane-mean and lane speed deviations have on highway safety. We propose a structural model that relates mean speed and speed deviations by lane and is contemporaneously influenced by environmental, temporal, and traffic flow factors. Spot speed and vehicle classification data measured by lane in both the eastbound and westbound directions of Interstate 90 (I-90) in Washington State are used to develop the empirical relationships. The findings show that lane-mean speeds are endogenously related with adjacent lane speeds and exogenously related with associated environmental, traffic flow and temporal factors, while lane-speed deviations are endogenously related not only with adjacent lane speed deviations but also, through forward causality, lane-mean speeds and exogenously related with environmental, traffic flow and temporal factors as well. The approach shows significant promise in unraveling cause-effect relationships affecting macroscopic traffic flow continuums.