A congestion mechanism for uphill expressways, Part I: the shoulder lane "release valve"

A mechanism is unveiled by which congestion forms and persists near the base of an uphill expressway segment, causing significant reductions in output flow. The traffic condition in the expressway's shoulder lane is key to the mechanism. When shoulder-lane flow was low, drivers maneuvered around speed disturbances that periodically arose in the median lane. The shoulder lane accommodated high rates of vehicle migrations, thus acting as a "release valve" for the excess accumulation created by the speed disturbances. The release valve failed only when demand increased later in the rush. The resulting higher flows in the shoulder lane impeded drivers' attempts to maneuver around the median-lane speed disturbances that occurred thereafter. These attempts disrupted traffic and spread the excess accumulation laterally across all lanes. When this queue filled the approach to the hill, vehicles arrived to its base at low speeds. This impeded vehicle ascent; output flow dropped by about 10%; and this state of affairs persisted for the remainder of the rush. The more conspicuous details of this mechanism were observed in loop detector data measured over many days at the site, and are consistent with observations previously made at other sites. The more subtle details became visible by examining thousands of vehicle trajectories extracted from a series of eleven roadside video cameras. Many of the subtleties are compatible with an existing theory of multi-lane traffic. All of this suggests that the present findings can be generalized to other uphill expressway segments. Practical implications are discussed.