Operational Performance Models for Freeway Truck Restrictions
Highways are designed to serve a mix of vehicle types—passenger cars, trucks, buses, recreational vehicles, etc. The impacts of these vehicle types, however, are not uniform and, therefore, pose special problems in highway operations. The increased operation of trucks in both volume and dimensions, coupled with the increase in passenger car volume, have added to existing problems related to traffic operations, safety, and roadway structures. In response, the Department has recently initiated several research projects in the related areas. Chief among them is a major Request for Proposals, issued this past November by the Safety Office, to study mainly fatal crashes involving trucks. In addition, the Systems Planning Office has recently embarked on two research projects, one to evaluate the potential for reserved truck lanes and truckways in Florida, and another to research issues related to truck LOS for FIHS facilities. These projects demonstrate a concerted effort by the Department to address the increasing impacts brought by truck traffic on the state’s roadway systems.
One common approach to reducing the impacts of truck traffic on freeways has been to impose certain restrictions on truck operations as a means of reducing the interaction between trucks and other vehicles and to compensate for their differences in operational characteristics. When properly implemented, truck restrictions can increase the overall operational efficiency of freeways and lead to improved traffic safety on these facilities. A variety of truck restriction methods have been implemented throughout the United States. Some use one lane while others use two lanes. Some use the inside lane(s) while others use the outside lane(s). Some use barriers to separate lanes for the exclusive use of trucks. Sometimes the lanes are restricted only during certain hours while others operate 24 hours a day. Some use a different speed limit for truck lane(s) while others do not. With the many possible design alternatives comes the need to identify the best-suited alternatives for implementation for specific prevailing conditions. A good estimate of the operational performance of candidate truck restriction methods will provide important input to making better planning and policy decisions.
However, the operational performance, usually measured in travel time, speed, density, capacity, number of lane changes, etc., of various truck restriction methods under a set of prevailing conditions cannot be easily estimated. The prevailing conditions can include truck and non-truck volumes, terrain type, interchange density, number of lanes, free-flow speed, etc. These quantitative models, which relate performance measures to the roadway prevailing conditions, will allow the expected performances of different truck restriction methods to be estimated and, thus, result in better decision-makings. Such models do not currently exist.
This project aims to develop operational performance models that can be applied to help identify the most operationally efficient truck restriction method on a freeway under prevailing conditions. The performance models will provide answers to such questions as under what levels of truck and non-truck volumes can an exclusive truck lane be justified and what are the expected travel speeds for truck and non-truck traffic before and after the implementation of a truck restriction method. With such models, a proposed truck restriction method can be evaluated before implementation, an existing method can be re-evaluated for possible improvements, and should a method become controversial, it can be defended objectively. The execution of this research project is in line with the increasing interest in addressing the problems brought by the increasing truck volumes on the Florida’s roadway systems.