Traffic Management

18 January 2011

Ever wondered why vehicles have to sit and wait at traffic lights but at other times there's a wave of green lights?

Council project manager with current traffic displays.

Council project manager with current traffic displays.

The delicate task of keeping vehicles moving in Wellington to minimise congestion at peak times and meet the needs of pedestrians is the responsibility of the City Council's traffic management centre.

A computer software package known as SCATS (Sydney Coordinated Adaptive Traffic Systems) allows us to monitor intersections and control traffic lights, to keep the traffic moving.

With 30,000 vehicles pouring into the Wellington CBD every morning between 7.00am - 9.00am, traffic flow must be carefully managed.

SCATS allows our traffic lights to adapt automatically to sudden changes in flow. We've found that fixed-time traffic light systems simply don't work as well as flexible systems when it comes to managing changing volumes of vehicle traffic.

"This system allows the traffic engineer to manage the operations of the transport network to keep it running as smoothly as possible" says Tim Kirby, the Council's signals project manager.

At the operations centre, monitoring staff use a bank of television screens to keep a watchful eye on critical parts of Wellington's network of roads and streets.

The SCATS system, developed in Sydney in the 1970s, is regularly upgraded with the latest software and is now used at 33,000 sites in 144 cities around the world. In tandem with CCTV cameras, SCATS is expected to remain our key traffic management tool for the foreseeable future.

But the benefits of SCATS go beyond providing green lights to keep traffic moving. SCATS gives our traffic engineers the means to monitor faults, reduce accidents and fuel consumption, and noise pollution is also reduced because vehicles do not have to stop and start so much.

SCATS can also give priority to public transport. At some intersections, for example, buses are given a few seconds head start on other traffic. This helps the buses change lanes.

SCATS also manages traffic associated with special events such as the Cuba Carnival, the Round the Bays Fun Run, movie premieres and protest marches. It is also invaluable when unplanned incidents occur. If needed, we can divert traffic away from overloaded areas or road blockages, and shepherd emergency services vehicles through the city with a "green wave" of traffic lights.

Tim says people sometimes ask why signals cannot provide a ?wave of green' lights to allow continuous movement of traffic.

"The reality is that there is a concurrent green signal time demand from multiple road users, including pedestrians, public transport buses, cyclists and cars," he says.

"SCATS adjusts this demand continuously by changing the amount of green time given to each type and direction of users. In other words, there are competing movements at every intersection. Some want to go straight ahead, some want to turn left or right and pedestrians want to cross.

"These movements must be managed to avoid crashes. SCATS effectively rations the available time at intersections for each movement while maintaining effective and safe flows of traffic."