The goal of this project was to establish spacing requirements for access points to managed lanes with respect to the location of entrance and exit ramps on the general purpose lanes of the freeway. Traffic entering the freeway destined for the managed lane must weave across the general purpose lanes. Traffic exiting the freeway from the managed lane must also perform this maneuver. The results are based on microscopic simulation using the VISSIM model. The simulation model was carefully calibrated using data collected on IH 635 (LBJ Freeway) in Dallas, Texas. A genetic algorithm was used in the calibration. The model was subsequently validated using data collected at a nearby site along IH 635. The weaving was analyzed as a Type C two-sided weave. Capacity was estimated by gradually increasing flow in the general purpose lanes for each set of conditions until the simulation model throughput was less than the input flows, indicating the formation of queues. The specific conditions included ramp flows (500 to 1250 veh/hour), ramp to managed lanes flows (100 to 400 veh/hour), general purpose lanes to managed lane flows (200 to 800 veh/hour), and length of weave (1000 to 4000 feet). The principal determinant for spacing was the weaving flow (ramp to managed lane flow), with a minimum weaving distance of 2000 to 3500 feet for flows from 200 to 400 veh/hour. A desirable minimum distance of 4000 feet was found. All results were for four general purpose lanes. (Author/publisher)
Abstract