Ask Auckland: How traffic lights work

Some days battling Auckland’s traffic congestion it feels like the traffic light phasing is shorter or every light you come to is red, but Auckland Transport says a closely monitored, internationally used, traffic signals system is keeping the city flowing.

Auckland’s traffic lights are controlled by a system developed in Australia called SCATS.

The same system is used in many countries around the world and has been continually developed over the past few decades, according to Auckland Transport spokesman Mark Hannan.

SCATS constantly collects traffic data and, every few minutes, allocates green-light time based on how busy each direction is. The system tries to find the best balance of wait times in every direction. It constantly adapts timings based on changing situations throughout the day.

‘‘Our engineers are trained in the same methods used by the engineers in Australia and we continually review the operation of our signals to ensure that they are operating efficiently,’’ Hannan said.

Pressure-based sensors at traffic lights stopped being used decades ago, Hannan said, and now approximately 99 per cent of sensors in Auckland are ‘‘essentially metal detectors’’ and are the main sensor used internationally.

‘‘These sensors do their job very well, at low cost and last a long time. There are several different types of sensors available and all have their various advantages and disadvantages. We have looked at infrared sensors in the past and these are only of use in limited situations. In practice, using infrared sensors would make little discernible difference to the operations of our traffic signals. At this stage, the current sensors we use are the most effective option.’’

North Shore driver Chris Marshall queried the efficiency of the congested intersection at Archers Rd/Sunnybrae Rd/Chivalry Rd in Glenfield.

‘‘These are a single set of signals that operate together. So they are coordinated in the sense that they operate as a single unit,’’ Hannan said. ‘‘The two side roads being offset by 60 metre means that this intersection will always run less efficiently than a standard crossroads intersection. The intersection essentially runs like a roundabout ... this is deemed the most efficient operation for this type of intersection.’’

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FELICITY REID/ STUFF Traffic stops at a red light in Birkenhead, Auckland.