INGRID is a real time automatic incident detection system which uses algorithms to detect incidents. Techniques have been developed to assess the effect of a detected incident on the network.
There are two algorithms which are used to detect incidents. One examines current traffic data for sudden changes in flow and occupancy. No reference data is required for this algorithm.
The other algorithm uses historic reference data provided by the ASTRID database. For all SCOOT detectors in the network a daily profile of the expected flow and occupancy in each 15 minute period is stored and automatically updated in the ASTRID database. The algorithm detects incidents by comparing the current traffic situation with that expected from the historic reference data in ASTRID. The algorithms use standard deviations and mean values to determine a confidence level against which to assess the current data. An incident is indicated if the conditions are satisfied for one minute. When the conditions are satisfied for three consecutive minutes more weight is given to the incident report.
Both algorithms require data on the flow and occupancy over loops on consecutive links to detect an incident in the road space between them. Incidents are indicated during the following conditions:
- decrease in occupancy and flow at the downstream detector
- increase in occupancy and a decrease in flow at the upstream detector
For best results the routine requires the flow and occupancy data for each traffic signal cycle.
Once an incident has been detected, it is important to establish the extent to which the incident will alter traffic flow around the network. There are two main effects to be considered - the area affected by the spread of congestion due to the incident and the additional delay to vehicles travelling through the affected area.
The severity index currently deployed in INGRID considers only the first effect by indicating those detectors which are affected by an incident. The greater the severity of an incident, the larger the number of affected detectors. INGRID calculates which detectors upstream and downstream of an incident are affected. In calculating the confidence of an incident, the duration of the incident is also considered.
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