Collisions in Freeway Traffic: Influence of Downstream Queues and Interim Means to Address Them

Findings from previous studies indicate that a freeway traffic collision is more likely to occur in close physical proximity to the tail of a queue. The implication is that collision likelihood increases when drivers abruptly alter their trajectories (e.g., by decelerating or changing lanes) on encountering the queue. The implication is supported and bolstered with new and detailed data that were painstakingly extracted from two freeway stretches in California. These data show how the likelihood of collision increases as both the spatial and the temporal proximities to the tail of an expanding or receding queue become smaller. It follows that collision risk may be reduced by instructing drivers to begin decelerating while still upstream of queues. Retarding vehicle progress toward a queue's tail could retard the rate by which the likelihood of collision grows with time. Having vehicles approach a queue at diminished speeds may diminish the expected severity of a collision, should one still occur. This information is considered in this development of the ideas behind simple control logic for issuing speed advisories to drivers as they approach a queue. This logic could assist freeway managers in selecting suitable advisories so as to diminish the empirical estimate of collision likelihood at a specified time in the immediate future and by some target amount. In the interim, the likelihood estimates would be based solely on vehicles’ spatiotemporal proximities to queues. Other influences would be ignored for now, although planned experiments to test and improve the present logic are discussed.