In Mpumalanga, only about 25 per cent of households have car access, which makes Mpumalanga one of the provinces with the highest use of public transport by means of bus or taxi in South Africa. Even though the road network in Mpumalanga is extensive, the maintenance and upgrade of this network is a concern, especially for municipal and provincial roads. Deteriorating roads have a direct (such as vehicle operating costs, VOCs) and indirect impact (such as high bus fares) on the road user.
This study focuses on the interaction between one bus from Buscor and the pavement surface of one specific bus route, including the associated VOCs generated. The ride comfortability of the bus route was evaluated by interpreting International Roughness Index (IRI), Power Spectral Density (PSD), and vertical accelerations (awz) data, after which the associated vehicle operating costs (VOCs) for the bus were projected. The judgement of ride comfortability in a vehicle is an area of controversy, and studies on this topic dates back from the 1920sThe threshold values from ISO (1997), Cantisani, and Loprencipe (2010) were used for the purposes of this study.
The vertical accelerations generated from the surface of the bus route, for a bi-articulated bus were measured with accelerometers. The accelerometers were placed on the bus where the vertical accelerations were expected to be the highest. The identified bus route included different roads with different responsible authorities and roughness levels. A profiler conducted a survey on the route and five different sections were identified. The collected data were analysed with various programs. From the data collected from the accelerometers the PSD, awz, and the speed that the bus travelled on each section could be determined.
The IRI data for each section was categorised in three categories, very good to good, fair to mediocre and poor. Anomalies in each section were identified, and the cause of these anomalies determined. The anomalies were analysed with the data, as these values formed part of the route.
The impact of road roughness on fuel consumption, tyre wear and repair and maintenance costs were analysed. The calibrated Highway Development and Management System (HDM 4) model was used to predict the fuel consumption, tyre wear and repair and maintenance cost per km of each section of the bus route under consideration.
In this analysis, the impact of the vehicle speed proved to be significant, as it affected the PSD values, the awz values and the VOC. The scenario was analysed to improve the riding quality of the two worst sections of the bus route, and by improving the road surface of these two sections, travel time could be reduced and costs could be saved. The scenario of an increase in road roughness was also analysed, to indicate the percentage of increase in costs, if these roads kept deteriorating. The analysis showed a significant increase in repair and maintenance cost, fuel consumption, and tyre wear.
The limitations of this study included some correlation issues with the profiler data and the accelerometer data, unidentified bus stops on gravel shoulders, and the suspension system and interior of the bus that were deemed constants and not variables within the scope of this study.
Despite these limitations, there are still a number of possibilities with the data collected. Apart for the analyses conducted for the study, recommendations for further refinement include the impact of bus mass on the data (full bus versus empty bus), the suspension type and condition, the interior of the bus, trip duration and congestion, evaluation of driver fatigue, tyre pressure and dynamic wheel loads, and safety. Furthermore, the construction cost of upgrading the two worst sections of the route versus the increase in VOCs because of the deteriorating state of the route could be investigated.
In conclusion, the speed played a determining role in the generation of vertical accelerations, therefore user comfort or discomfort, and VOCs. An increase in IRI indicates deteriorating riding quality that affects the comfortability of a ride and the VOCs negatively. IRI can be used to determine the state of the road and qwz gives an indication of the comfortability of a ride.