Ngauranga Gorge
Incident Detection
Variable message signs in Ngauranga Gorge
Large gantry supporting signs (click to enlarge)
Bottom of Ngauranga Gorge
This method can be efficient; but it depends on the normal traffic flow being consistent, and tends to fail when the traffic is either very heavy or very light. The major drawback of the method is that it can take as much as 5 minutes to detect an incident.
During the preparation of the design proposal, it was recognised that the traditional method of incident detection would not be effective in the Ngauranga Gorge, and would be far too slow for oncoming motorists to be warned of a hazard. Techniques based on video cameras and video image processing were proposed.
Video image processing directly detects stationary vehicles and other objects anywhere within the field of view. Coverage includes through lanes, ramps and shoulders, totalling 95% of the overall area of the Ngauranga Gorge highway. Processing also reports the current congestion level and detects the sudden formation of the queues that often results from incidents.
Unlike traditional methods, video image processing detects not only actual incidents, but also hazards before they become active incidents. Reporting is specific as to the location and nature of the incident and is accompanied by immediate video coverage of the area.
Most importantly, incidents are detected within 15 seconds. The combination of rapid detection and accurate information provides vital input to the system operator responsible for managing and responding to incidents.
Incident Response Tools
When incidents are detected, oncoming motorists are informed of the hazard by six Variable Message Signs, located at strategic points on the approaches to and in the Gorge. Hazards may include lane closures for roadworks, poor visibility or slippery surfaces detected by the two weather stations, and concealed queues at off ramps.
Variable Mandatory Speed Signs can slow traffic to a safe speed. The signs are electronic versions of standard speed-limit signs, and posted speeds are legally enforceable. The same 23 signs can also be used to inform motorists of lane availability, though this option is not currently used by Transit.
Police have access to the same Highway Status information as the operators, including currently displayed messages, speed limits and incident information. They also have access to real-time video images from the fixed incident detection cameras and the pan/tilt/zoom (PTZ) cameras. They can therefore despatch an appropriate response team as soon as an incident is detected, and can brief the team whilst enroute. The result is a significant reduction of the overall duration of incident and of consequent congestion.
