Due to unreasonable combination of length and grades of slopes, the number of traffic accidents on long and steep slopes is increasing rapidly in China. The paper aims to explore speed performance of heavy duty vehicles on slopes after analysis of the forces distribution during uphill and downhill procedures. A speed gain model is developed with various power-to-weight ratios ranging from 5 kw/t to 8 kw/t for the uphill and downhill process respectively. The study results show that on a certain grade, a higher power-to-weight ratio results in a smaller speed reduction and a longer equilibrium slope during uphill process before finally reaching a higher stable speed, whereas during the downhill process on a certain grade, the higher power-to-weight ratio of the vehicle results in a dramatic vehicle speed increases and a longer distance to reach the stable operating speed. The critical lengths of upgrades and downgrades are identified if speed gaps are over 20 km/h between adjacent sections. Finally, these results are applied to set climbing lanes and escape lanes to improve safety, and provide smooth traffic operation in mountainous areas. The results can also provide guidelines for roadway geometry design as well.

Traffic safety; Critical length of grade; Speed profile; Power-to-weight ratio; Heavy vehicle