TRAFFIC CHARACTERISTICS ON 2-LANE HIGHWAY DOWNGRADES

Downgrade operations are not specifically addressed in the 1985 Highwa y Capacity Manual procedures for two-lane highways. However, traffic o perations on long steep downgrades on two-lane highways are becoming i ncreasingly important due to increasing volumes and the higher percent of slow moving vehicles such as recreational vehicles and heavy truck s in the traffic stream. Compounding the slow moving vehicle platoonin g problem is the fact that most downgrades on the primary highway syst em in Western Canada are long no-passing zones. It is noted that most upgrades on the primary highway system have climbing lanes, and passin g lanes are being built on level tangent sections which have extended no-passing zones. This paper presents the findings of a study of traff ic how on long downgrades on the primary highway system in Western Can ada. Downgrade data was collected on three long, steep downgrades usin g a time lapse video camera. The findings indicated that while truck s peeds on level terrain are only slightly slower than passenger cars, o n downgrades the presence of trucks noticeably affects speed-few relat ionships. Both the Hyperlang and Schuhl headway distributions produced excellent fits to the headway data. The data yielded over 6000 platoo ns. The Geometric, Borel-Tanner, and one parameter Miller distribution s provided a good representation of traffic flow on downgrades only un der some conditions, however, the two-parameter Miller distribution pr oduced very good fits in all cases of platoon size distributions.