4.2 Traffic Monitoring
A traffic monitoring capability is at the heart of any TMC. Systems deployed to provide this capability will depend largely on the types of road in the network being monitored.
On surface arterial roads with at-grade intersections controlled by wide area traffic signal systems, traffic signal loop detectors provide a vehicle detection capability enabling some systems (SCATS and STREAMS for example) to identify traffic congestion levels on intersection approaches. Figure 4.3 shows a SCATS display of congestion levels.
Source: Roads and Traffic Authority NSW (2000).
Congestion occurring at an unusual time and/or location can indicate the occurrence of an incident. In SCATS the capability to detect this situation is called the Unusual Congestion Monitor.
On motorways or freeways, detectors (in Australia and New Zealand usually loop detectors, but other technologies may be used) provide a first line monitoring capability. For example, automatic incident detection (AID) systems are available to analyse detector data and identify potential incidents. The major issue for TMC operators is the balance between time to detect, detection rate and false alarm rate. An AID system tuned for rapid detection with low probability of ‘missing’ an incident will also have a high false alarm rate. Means of incident verification are therefore an essential adjunct to detection systems.
Closed circuit television (CCTV) is the most commonly deployed monitoring technology capable of incident verification. CCTV cameras with pan-tilt-zoom capability are normally deployed on major traffic routes, at critical intersections and at hazardous locations such as tunnels. Their role in direct incident detection is limited, but their role in incident verification is essential and they can be useful in monitoring progress during incident investigation and clearance if they happen to be located near the site.
CCTV cameras may be permanently on-line, or dial-up communications may be employed to connect them only when needed. A range of communication technologies with varying bandwidth may be used, with picture quality varying as a result.