| Summary: | To mitigate the effects of non-recurrent congestion on the United Kingdom motorway and trunk road network, the Highways Agency (HA) has developed the Incident Support Unit (ISU) service. This research presents a review of incident management practice in the United Kingdom, in particular on the M25 London Orbital Motorway. An international comparison between British incident management operations and those in the United States is also provided. The ISU service on the M25 motorway, operated by the HA’s service provider, Carillion plc, is critically examined, including quantitative and qualitative examinations and a benefit-cost estimation. To understand fully the influence that ISUs have on the M25 road network motorway, incident data was collected and analysed. These incidents have been examined to determine their influence on traffic flow. Specifically, their impact on the capacity of the roadway and the effect of “rubbernecking” is investigated. Investigations and analysis are undertaken to evaluate the delays experienced by motorway users due to incidents. The effectiveness of motorway matrix signals and signs are then examined including compliance rates with mandatory signals and the impact of variable message signs on driver route choice. Finally, the optimal standby locations of ISUs on the M25 sphere road network are established in order to reduce their response times to incidents.
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