Traffic impact assessment of incident management strategies

This paper presents results from a simulation study which aimed to quantify the impacts of incident management strategies. The evaluation was based on a large-scale micro-simulation model covering an area approximately 122 square kilometres, including 43 kilometres of motorway and about 85 kilometres of surface roads on the Gold Coast, Australia. The study examined the effectiveness of selected incident management strategies including ramp metering, VMS information dissemination combined with route diversions, and variable speed limit systems. The provision of VMS information on the motorway and dynamic adjustment of signal timings on the diversion route resulted in equilibrium conditions and balanced distribution of traffic on both the normal and diversion routes when the optimal diversion rate was 30 percent. This resulted in reduction of delays by 8.8 percent, decrease in number of stops by 22 percent, and decrease in travel times by 3.3 percent. An important finding was that these benefits are only realised when the two incident management responses (VMS route diversion and dynamic traffic signal plans on surface roads) are implemented at the same time. Combined, their impact was such that they resulted in restoration of traffic conditions to the pre-incident situation. The paper also reports on a preliminary investigation into variable speed limits (VSL) as a means to reduce the negative impacts of incidents. The results showed that VSL had the potential to provide an 11 percent improvement in efficiency and also contributed to improving safety by homogenising the flow in higher speed regimes. The number of stops per vehicle was found to decrease by 64 percent as a result of reducing the speed limit from 110 kph to 70 kph due to an incident on the motorway. The results reported in this paper show that the impacts of incident management strategies are network-dependent, and that these strategies, when combined, have the potential to provide substantial economic benefits in terms of reduction in travel times, and improvements in safety conditions