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Major intersections in the northern suburbs have been identified as having WA's longest red-light waiting times.

Motorists at the Scarborough Beach Road and West Coast Highway intersection in Scarborough could wait up to 210 seconds for the red light to change to green - the longest waiting time in WA.

The State's shortest traffic light cycle is at the corner of Cathedral Avenue and Chapman Road in Geraldton, where the maximum wait time is only 80 seconds.

The waiting times have been revealed as part of a special briefing provided to _The West Australian _ by Main Roads WA.

But as traffic operations and services manager Craig Wooldridge indicated, there are a number of factors that could influence waiting times at traffic lights.

Traffic lights are in use at 882 intersections around WA, including 831 in the metropolitan area.

Each set of lights is allocated a maximum cycle time, based on traffic flow during peak periods.

But a computer system being used by Main Roads WA allows this time to change throughout the day, depending on traffic volumes.

The system has the ability to measure the number of cars using the intersection and the space between each car - an indicator of congestion.

As a result, the cycle time at a basic intersection with no arrows could drop to as low as 36 seconds when there is little traffic.

The signals could also be changed to accommodate special events, for example, at the conclusion of major sporting events, or for health emergencies, for example, the transfer of a donated human organ from the airport to the hospital.

Mr Wooldridge said the computer system known as SCATS - Sydney Coordinated Adaptive Traffic Signals - was introduced in Perth in 1983 and has gradually been connected to all sets of traffic lights.

He said it allowed Main Roads to monitor congestion and to make adjustments to improve traffic flow.

In addition, Main Roads has about 120 cameras around the city that are monitored throughout the day.

"Information we receive from SCATS and the cameras provide us with a fairly comprehensive picture of traffic flows around the metropolitan area at any time of the day," Mr Wooldridge.

"I suspect it won't be very long before this information is available to motorists through an in-car facility that will allow them to be aware of traffic hot spots and to avoid them."

Main Roads traffic systems operations officer Martin Woolley said motorists were often frustrated at waiting at a red light for a seemingly unnecessarily long time.

But he said traffic signals in a co-ordinated route operated on the same cycle length and the delay might have been necessary to ensure traffic lights on an arterial road remain synchronised.

"An analogy to explain this concept is a bike with gears connected by a chain," Mr Woolley said. "As long as the gears are the same size, they will always remain in step.

"With provision made for the distance between intersections, it provides motorists with a green-light run on a given route at a given time.

"Of course, if you are driving against this flow, it may also mean a run of red lights."