Abstract
Dedicated Truck Lanes (DTL) are planned in the United States along freight-intensive corridors, namely Corridors of the Future. In rural areas along these corridors, slip ramps will provide access to heavy vehicles between the truck-only lanes (TOL) and the general purpose lanes (GPL) to subsequently enter and exit the facility using an interchange. This paper examines the distance between the slip ramp and an interchange required by heavy vehicles to enter and exit the facility using a microscopic traffic simulation program, VISSIM. This distance consists of a merge, link, and a diverge segment. The procedure for determining this distance included (a) examining the acceleration/deceleration characteristics of vehicles, (b) specifying the parameters in VISSIM based on vehicle characteristics and driver behavior, (c) coding and simulating the traffic on DTL in VISSIM, (d) analyzing the speed–flow relation for passenger and heavy vehicles for the various lengths of merge, link, and diverge segments on the GPL, slip ramps, merge, and diverge segments of TOL, and (e) determining the slip ramp distance to (and from) the interchange for level, upgrade, and downgrade. To determine the different segment lengths, a rotatable central composite design (RCCD) was used to generate a reasonable number of cases required for simulation. Simulation of peak period traffic on level grades indicated that the lengths of segments were directly proportional to the average speed of heavy vehicles. As the slip ramp distance from and to the interchange depended on the lane change behavior of drivers, a sensitivity analysis was performed for the safety distance reduction factor (SDRF), the main lane change parameter in VISSIM. This paper provides state departments of transportation with design recommendations for slip ramp design for the Corridors of the Future project. The paper also provides a unique application of microscopic simulation to highway facility design.
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