Bridge lights will help drivers go with the flow

By ALAN PERROTT

Rush-hour motorists will be drip-fed on to Auckland's motorways by an electronic sign system if a planned Transit New Zealand trial on Mangere Bridge is successful.

The road building agency is investigating setting up a new traffic light network on the Rimu Rd onramp to control the flow of vehicles trying to get on to the bridge during peak times.

Mangere Bridge carries about 65,000 vehicles a day, but the flood of cars merging from the onramp slows average speeds from 85km/h to 30km/h, backing up traffic for several kilometres to the south.

Ramp metering, already used on sections of Melbourne's motorway network, controls the number of vehicles moving on to the motorway.

Electronic signs will be set up some distance ahead of the traffic lights warning drivers when the system is working.

The lights are activated when sensors show traffic on the bridge and the onramp is heavy.

Vehicles will then form up along two lanes on the northbound onramp with one traffic light controlling each lane.

Both lights will go red and stop all traffic on the onramp, then one will go green for a few seconds to allow one car to merge.

When that light returns to red, the other light will go green and allow one car from the other lane to move forward.

This system can lead to increased queues waiting to use the onramp, but a short trial on the Esmonde Rd onramp to the Northern Motorway several years ago showed it improved overall flow.

Installation of the lights, signs, sensors and closed circuit television monitors will cost Transit New Zealand $300,000.

The trial was approved by the Transit New Zealand Authority in December and has the support of the Manukau City Council, subject to the development of suitable communications, monitoring and enforcement systems.