Downgrade Passing Lanes on the Canadian Trans-Mountain Highway System

Traffic operations on long steep downgrades on two-lane highways in mountainous terrain in Western Canada are becoming increasingly important from a safety and levelof-service perspective due to increasing volumes and higher percentages of heavy trucks and recreational vehicles in the traffic stream. Most steep upgrades on the primary transmountain highway system have been provided with climbing lanes. However, all climbing lane sections have a solid double centre-line (based on the AADT criteria of 4000) that prohibits passing in the downgrade direction. Many trans-mountain downgrades are long and steep enough (grades of 6% and longer than 6 km) that heavy vehicles must travel at crawl speed to avoid loss of control. The low speed combined with no passing zones over extended sections results in long platoons and a low level-ofservice. Safety issues arise when frustrated drivers attempt to overtake. The Highway Capacity Manual (HCM2000) provides a procedure for determining passenger car equivalents for estimating the effect of heavy trucks operating at crawl speed on long, steep downgrades. However, geometric design guides such as the TAC Geometric Design Guide for Canadian Roads and the AASHTO Policy on Geometric Design of Highways and Streets do not provide guidelines for the planning and design of downgrade passing lanes or how to combine climbing lanes and downgrade passing lanes. This is considered a critical highway design issue as steep grades are located in difficult terrain where engineering a wide cross section is both challenging and costly. Parks Canada has constructed downgrade-passing lanes on the Trans Canada Highway in the Kicking Horse Pass in Yoho National Park and on Beaver Hill in Glacier National Park. These downgrade-passing lanes complement the passing lane system on the Trans Canada Highway in the Mountain National Parks. The purpose of this paper is to report on the experience of Parks Canada in designing, constructing and operating downgradepassing lanes. The paper also offers suggestions as to how climbing lanes and downgrade passing lanes could be combined on two-lane highways in mountainous terrain.