Effect of grade and climbing lane length on performance of heavy trucks on two lane rural roads in Uganda

Abstract

The grade and length of climbing lane affect performance of heavy trucks, leading to significant speed reductions, safety risks due to increases in passing manoeuvres and a reduction in the quality of traffic flow due to delays. The effect of grade varies amongst vehicle categories due to variance in their physical characteristics. The geometric design standard adapted in Uganda is based on design thresholds set in AASHTO Green Book (2011) in which climbing lanes are required where trucks experience speed reductions of at least 20km/h. The study sought to assess heavy trucks performance on climbing lanes by grade and climbing lane length. Data was collected from eight sites in flat and rolling terrain with vertical grades in the range 2-9% and length of climbing lanes in the range 450-1800 meters. Vehicle speed data was collected using MH 350 Corbin traffic classifiers placed at the beginning, two intermediate positions and at the end of the climbing lane. The results show that upgrades of 2% and 3% have a slight but not statistically significant effect on the truck speed reduction. However, grades in the range 4-9% showed statistically significant speed reductions for trucks. The critical lengths of grades for which a 20km/h speed reduction is realized for 2%, 3%, 4%, 5%, 6%, 7%, 8% and 9% were 850, 400, 300, 240, 240, 130, 80 and 50 meters, respectively. The critical lengths of grade were much lower than assumed in design necessitating a need to update the design guide to suit local conditions which are accordingly recommended in this study. In conclusion, this study determined that the curves adopted in MoWT (2010) were not reflective of the actual performance of heavy trucks traversing on rolling and flat terrains on two-lane rural highways in Uganda.