Influence of rainfall on the capacity of single carriageway in Malaysia

Abstract

Traffic flow disturbances come from intersections, on/off ramps, work zones, pavement distress sections, tapering sections and ambient conditions. These cause speed drops and traffic flow rate changes and impact on the traffic carrying ability of roadway sections. These disturbances generate queues, cause long delays, increase travel times and may result in accidents. All these causes and effects must be considered if highway facilities should be operated unhindered. Thus this thesis examines the problems of rainfall disturbance to traffic flow and highway capacity. The effect of rainfall is more severe in tropical regions where it occurs in large quantities throughout the year. Thus four sites were set up in east and south Malaysian Peninsula respectively to generate data for the study. Data was collected using automatic traffic counters and rainfall data was obtained from two surface rain gauge stations that were 1174m and 1840m away from the data collection sites. The empirical data pointed towards speed and traffic flow reduction during rainfall spells. Further analysis using the fundamental diagram (trapezoidal flow contraction) method confirmed speed and flow reduction as the main impacts of rainfall to traffic flow in adverse weather. For the scenarios tested, such as peak period versus nonpeak, the trend remained the same. However, peak hour flow conditions are associated with instabilities that are difficult to separate from rainfall disturbances and were not analysed further. Highway capacity drops occurred between the dry condition and the three rainy regimes tested. Implications of traffic flow contraction for Passenger Car Equivalency (PCE) values of vehicles and traffic shock waves propagation were also investigated. PCE of heavy vehicles were lower than the values employed in the Arahan Teknik and the Malaysian Highway Capacity Manual, suggesting better performance of these vehicles under rainfall conditions mainly due to larger average headways and the prevailing free-flow conditions. In the case of traffic shock wave, the wave speeds were all lower than the speeds at critical density and the wave generated moved principally in the direction of the main stream flow, suggesting rarefaction waves rather than shock waves. The findings in this study could be incorporated into a wider strategy (Intelligent transportation system) to inform and assist drivers in inclement weather.