Traffic characteristics on two-lane highway downgrades

Downgrade operations are not specifically addressed in the 1985 Highway Capacity Manual procedures for two-lane highways. However, traffic operations on long steep downgrades on two-lane highways are becoming increasingly important due to increasing volumes and the higher percent of slow moving vehicles such as recreational vehicles and heavy trucks in the traffic stream. Compounding the slow moving vehicle platooning problem is the fact that most downgrades on the primary highway system in Western Canada are long no-passing zones. It is noted that most upgrades on the primary highway system have climbing lanes, and passing lanes are being built on level tangent sections which have extended no-passing zones. This paper presents the findings of a study of traffic flow on long downgrades on the primary highway system in Western Canada. Downgrade data was collected on three long, steep downgrades using a time lapse video camera. The findings indicated that while truck speeds on level terrain are only slightly slower than passenger cars, on downgrades the presence of trucks noticeably affects speed-flow relationships. Both the Hyperlang and Schuhl headway distributions produced excellent fits to the headway data. The data yielded over 6000 platoons. The Geometric, Borel-Tanner, and one parameter Miller distributions provided a good representation of traffic flow on downgrades only under some conditions, however, the two-parameter Miller distribution produced very good fits in all cases of platoon size distributions.