Key Points:

Traffic lights will begin regulating vehicle flows on to Auckland's Southern Motorway on Wednesday, starting at Papakura before extending to 31 on-ramps between there and the harbour bridge by August.

A further 30 ramps should be similarly controlled on the Northwestern and Northern Motorways by the end of 2008, in an elaborate traffic management project expected to cost Transit NZ around $50 million.

The first set of what Transit is calling ramp signals to go "live" will be at Papakura's southbound motorway entry point, to be followed by another pair upstream at Takanini next Monday.

Others to be activated before Christmas will be along the northbound ramps from Wellington St and from Curran St to the harbour bridge, and on long-awaited new links due to open on December 18 and 19 in the same direction from the port and the Northwestern Motorway.

The signals will look like normal traffic lights and be linked to sensors embedded in road surfaces, which will measure vehicle flows to determine times for their red and green phases.

Running a red signal will hold the same $150 penalty as driving through any other traffic light, and Transit says the police will monitor motorway ramps with closed-circuit TV cameras.

Vehicles will be held back when the lights are switched on at peak periods, before being released one-by-one to merge with mainstream motorway traffic at measured intervals.

Transit acknowledges this may lengthen delays at the on-ramps by several minutes or more at busy times, but predicts "a small wait for a greater gain".

"Regulating flow on the on-ramp helps to relieve congestion and faster speeds can be reached on the motorway as a result," the agency says. It also expects fewer crashes caused by merging traffic or by stop-start vehicle movements along what it hopes will be a smoother-flowing motorway.

But the agency expects some extra impact on local streets and is even inviting motorists to consider staying off the motorway for shorter trips, or using public transport.

That will put Auckland's city and district councils on alert, even though they have secured a written agreement from Transit to modify the timing of ramp signals or even turn them off if they cause unacceptable queues.

Transit also intends installing signal-free lanes for trucks and high-occupancy vehicles at several on-ramps which climb to reach the motorway, so they will not hold up other traffic by having to stop and start.

But Auckland Regional Land Transport Committee member Chris Carr, of road haulage firm Carr and Haslam, fears traffic may queue too far back to allow trucks to reach fast lanes.

That meant frustrated truck drivers might start looking for faster routes through local streets.

Transit regional manager Peter Spies promises that ramp signals will be monitored closely by traffic management centre staff using "state-of-the-art" technology.