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, conditions, however, the two-parameter Miller distribution produced very good fits in all cases of platoon size distributions. (A)
Abstract