Study of Long Combination Vehicles on Two-Lane Highways

In 1988, the Council of Ministers of Transportation and Highway Safety in Canada endorsed a memorandum of understanding designed to improve uniformity between provinces and territories. The regulations covering weights and dimensions for tractor-semitrailers and double combination trucks specified a maximum length of 23 m. However, in western Canada, which includes the provinces of British Columbia, Alberta, Saskatchewan, and Manitoba, a maximum vehicle length of 25 m was adopted. Vehicles longer than 25 m are generally referred to as long combination vehicles (LCVs) and include the following vehicles: triple trailer combinations, 35 m in length; Rocky Mountain doubles, 30 m in length; log haul trucks, which can be up to 30.5 m in length; and turnpike doubles, 38 m in length. A study was conducted to develop recommendations relating to the use of LCVs on two-lane highways in Alberta. The basic approach was to examine the incremental impact of LCV-type vehicles on two-lane highway operations compared with a baseline vehicle type, namely, 25-m standard double heavy trucks. Although the study of LCVs examined a range of issues that included low- and high-speed offtracking, and aerodynamic buffeting, only the findings related to traffic volume criteria are presented. The main recommendation with respect to passing opportunities and level of service was the development of maximum traffic volumes for two-lane highways with and without passing lanes and with a given percentage of passing zones. The main criteria are to ensure that a net passing opportunity of at least 30 percent is provided to drivers impeded by LCVs. This is roughly comparable to providing to Level of Service C. For example, the maximum volume below which Rocky Mountain doubles may operate on a two-lane highway is 425 veh/h for a section with 100 percent passing zones. If 2-km passing lanes are provided every 10 km, Rocky Mountain doubles could operate at traffic volumes up to 734 veh/h and still ensure a net passing opportunity of 30 percent.