Investigating the Effect of Bituminous Pavement Roughness on New Vehicle Parts Consumption

Abstract

Kampala has a total road network of 2,110 km, with 575 km (27.4%) of paved roads and 1,535 km (72.6%) of unpaved roads, with over 300 vehicles per day. With the ever-increasing traffic volumes, maintaining them in a paved state is becoming too expensive and unsustainable. While there has been significant progress in improving the network, with total roads classified as good condition increasing from 8% to 22.4 % and those classified as fair condition expanding from 17% to 41.6 % over a 5 year period, a significant portion of the network remains in poor condition (KCCA, 2016). Road development continues to consume the majority of the budget, accounting for 91% of the total, while road maintenance accounts for 7%. This imbalance is causing the network to develop in an unsustainable manner, and the growing maintenance backlog is expected to result in greater road asset replacement costs in the future (UNRA, 2020/21). The poor road network leads to increase in the costs borne by the users of the road such as; lubricant consumption, tyre ware, travel time, fuel consumption and vehicle parts consumption. Poor road conditions contribute immensely to the condition of the vehicles, hence the need to establish the effect of the road condition on road user costs such as vehicle parts consumption. This study investigated the effect of bituminous pavement roughness on new vehicle parts consumption. Ten (10) bituminous roads under the authority of KCCA with different road conditions ranging from excellent to bad were selected. The study comprised of both primary and secondary data collected through surveys, actual field observations, measurements, and the cost of brand new vehicle prices. The defects along the selected roads were quantified, the IRI was estimated, and then the relationship between the IRI and new vehicle parts consumption was established based on an equation by the Brazilian HDM III model. Six (6) vehicle categories comprising of passenger cars, utility vehicles, large buses, light and medium trucks, heavy trucks, and articulated trucks were selected. From the study, utility vehicles and passenger cars were the most sensitive to changes in road roughness, especially above 6 IRI m/km, while large buses were the least sensitive. An increase in roughness from 2 to 10 IRI m/km (i.e. for a road condition change from good to poor), resulted in an increase in parts consumption by a factor of 4.1 for passenger cars in comparison to only 1.4 for large buses.