Ngauranga Gorge

Techlink would like to thank the experts at Transit New Zealand, Serco, Tyco and MWH (Montgomery Watson Harza) New Zealand Ltd, for their contributions to this case study.

Introduction

In February 2001 Transit New Zealand began operation of an internationally leading-edge Active Traffic Management System (ATMS) within a 4km section of the Ngauranga Gorge between Johnsonville in Wellington and the SH1 and SH2 interchange.

The Ngauranga ATMS is designed to reduce the number, severity and impact of incidents on this demanding section of highway and improve overall traffic flow (Conceptual statement). This has been achieved through the communication of up-to-the-minute information to road users, emergency services and traffic controllers and implementation of faster, more appropriate traffic and emergency management responses. The system is intended to reduce the number of accidents, driver frustration and delays by creating a safer motorway environment.

Overview

The Ngauranga ATMS is a sophisticated electronic system, controlled by computer stations.

The computer stations operate a network of:

• variable message signs;
• variable mandatory speed signs;
• pan-tilt-zoom (PTZ) cameras;
• fixed cameras (including automatic video incident detection and traffic flow monitoring for the Ngauranga Gorge, and smoke detection for the Mt Victoria and Terrace Tunnels);
• weather monitoring stations.

Sophisticated computer software (named Odyssey) co-ordinates the network.

The Ngauranga ATMS includes a video detection system and alarm manager that automatically alerts the operator when an incident has occurred. Traditionally these systems require human operators to scan banks of video screens to detect incidents.

The software automatically detects traffic incidents (for example stationary vehicles) triggering an incident alarm and initiating an appropriate response strategy.

The advantage of this system is the need for only one video screen to monitor the entire coverage area. The detection system raises the alarm and automatically connects with the appropriate video camera in order to display the relevant video image. This means immediate identification of incidents enabling faster, more accurate responses.

Previously emergency services had no access to reliable information on the nature, location and seriousness of incidents in the Ngauranga Gorge until a response unit arrived at the scene. Access to reliable information can now be provided over the entire detection area via up-to-the-minute video images.

Challenges

This project involved a number of complexities and challenges including a very tight timeframe for design, construction, testing and commissioning (approximately 15 months).

The Ngauranga ATMS was a pilot project. No off-the-shelf products were available to meet Transit's requirements, new technology had to be developed and tested.

Economic and Social Aspects

A development that is being investigated is the linking of key video cameras to a website, allowing the public to check on driving conditions along the route at any time. It's anticipated that this will be used by people especially around holiday weekends and major events.

Environmental issues

There was a relatively minor level of construction involved and environmental considerations were not a major issue. Potential adverse effects were identified and the following actions taken:

• The overall number of new structures (including gantries and poles) was reduced by strengthening existing structures and placing others at strategic locations. This reduced the quantity of raw materials used, and also adverse visual affects for drivers.
• The new structures were designed to be in keeping with the existing structures and sign posts in the Ngauranga Gorge. This provides an aesthetically pleasing and consistent environment for drivers.
• The fibre optic cable connecting the system components was installed in compliance with Transit New Zealand's Environmental Code of Practice. This ensured that trenches were dug during dry weather to minimise the risk of run-off.
• All cabling was laid underground to reduce the overall visual impact.

Incident Detection

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.