Three new speed cameras have been proposed for the Wellington motorway, in addition to the one at Ngauranga Gorge.

Multiple speed cameras are likely to be installed along Wellington's motorway as part of plans for a "smart" road on which computers could cut limits to as low as 40kmh.

The New Zealand Transport Agency wants three fixed speed cameras installed between Aotea Quay and the Ngauranga interchange once an $87 million project to create a fourth northbound lane is complete in April 2016.

The project will also transform the road into the country's first "smart motorway", where speed limits are controlled by computers.

NZTransportAgency/YouTube 'Traffic scientist' Kevin McPhee explains how Wellington's "smart motorway" upgrade will speed up your journey.

The aim is to prevent gridlock at choke points such as the Terrace Tunnel by slowing traffic further along the road.

From April next year, motorists will encounter electronic speed signs and message boards every 500 or so metres along the motorway between Johnsonville and the Terrace Tunnel.

Sensors under the road, and radars mounted on light poles and gantries, will count the number of vehicles in each lane, as well as the speed they are travelling.

ROSS GIBLIN/Dominion Post Engineer Jack Gillard explains how an old off-ramp at Kaiwharawhara was pushed sideways to join with the Wellington motorway. The off-ramp will be 'stitched' on to the motorway to create a fourth northbound lane.

If the central computer thinks any part of the motorway is close to becoming congested, it will reduce the speed limit for approaching traffic to stop it from happening.

"So the limit might drop down to 80kmh, then down to 60kmh, then maybe down to even 40kmh," smart motorway project manager Glen Prince said.

The cameras would be there to ensure everyone reaped the time-saving benefits. "It's not about issuing infringement notices. "We want to create an environment that encourages compliance, but for those who won't comply, then there's a little bit of a stick at the end."

But the Automobile Association, which represents more than 1.4 million motorists, is not keen on the idea of speed cameras being used for control traffic flow rather than for safety reasons.

"Rushing into enforcement would be a big mistake in terms of building trust with the public," motoring affairs general manager Mike Noon said.

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Prince said the smart motorway system would deliver more reliable travel times, fewer carbon emissions, less congestion and less stress for motorists – but compliance was crucial.

"Some will need to get an infringement notice before they actually start behaving in an appropriate manner."

NZTA said current compliance with motorway speed limits was not good, and a recent survey of 222 Wellington drivers revealed almost 30 per cent thought the electronic speed signs were a suggestion, rather than the legal speed limit.

Noon said it would take time for motorists to get their heads around the idea of speed limits being reduced when a road did not appear to be clogged, and he was concerned about the variable limits being enforced from day one.

"If 30 per cent of the public think electronic speed signs are just a suggestion, then that means 30 per cent of the public could end up getting a speeding ticket."

MOTORWAY GETTING WIDER

Wellington's new-look motorway has already passed a major milestone.

The final piece of an old off-ramp at Kaiwharawhara was shunted sideways on Tuesday to help create a fourth northbound lane.

Contractors used hydraulic jacks to push the 300-tonne, 30-metre-long section. It took about an hour to shift it half a metre.

On August 1, concrete will be poured into the gap, fusing the off-ramp and the motorway together.

The overnight works will force the northbound lanes of the Thorndon overbridge to close for up to 12 hours.