Investigating the Effect of Road Gradient on Accident Occurence at Signalised Junctions in Kampala

Abstract

Road gradient is a geometrical design factor to be taken into account with road traffic accidents since it has been established that it can have an effect on the accident rates at signalized junctions. This is mainly due to the failure of heavy good vehicles to break at ascents and descents at approaches to signalized junctions. The main aim of this research was to develop a deeper understanding of the effect of road gradient on accident occurrence at signalised junctions in Kampala. A way to improve road safety is improving the road gradient to mitigate accident occurrence and severity at signalised junctions. In order to improve the road gradient, it is crucial to investigate and define the relationship between accident severity and road gradient. In order to study the effect of road gradient on accident severity at signalised junctions, accident data for 2018, 2019 and 2021 was collected from Kira Road Police Station for seven signalised junctions and the accident severity index was then determined using the equation based on the classification of accident as fatal, minor and serious. The concept of this method is that the fatal or serious accidents are given greater weights than the minor accidents. The weights are given based on social economic values. The equation used to calculate accident severity index was; where WSI is accident severity index, K is the number of fatalities, GI is the number of serious injuries, MI is the number of minor injuries (Binuu, 2018). The road grade was also measured along the different junction arms using a Global Positioning System with some arms being flat, ascents and descents. Generally, both steep ascents and descents affect accident severity of signalised intersections. The accident severity increases with increase in road gradient at both ascents and descents. However, it is preferrable to have a steep descent than a steep ascent at a signalised junction approach since the severity of the accidents at a steep descent is about 3.57 times less than that at a steep ascent. For every 50 increase in ascent, the accident severity increases by 18.38 which is equivalent to an increase of about 4.48 serious accidents while for every 50 increase in descent, the accident severity increases by 5.15 which is equivalent to approximately 1.26 serious accidents. This is as a result of the ease for heavy goods vehicles to come to a stop at a steep descent rather than a steep ascent.