Help for on-ramp mergers

Some Auckland motorway on-ramps will have to be modified to let traffic merge faster in a project to "manage" travel demand by rationing congested road space.

The $48 million Transit New Zealand plan includes installing traffic lights on each of Auckland's 120 or so on-ramps, to enable motorways to accept merging vehicles with less disruption to mainstream flows.

Project director Peter McCombs told Auckland Regional Council's transport committee yesterday that about 30 sets of lights would be fitted this financial year, and another batch would be commissioned late next year among central motorway junction improvements.

Despite continuing criticism by some Mangere Bridge residents about a "ramp-metering" trial on their entry to the Southwestern Motorway, Mr McCombs said it had improved overall speeds by 11 per cent to 30 per cent while increasing traffic flow.

He said it was not just a matter of reducing travel times, but of making them more consistent, to allow people to plan their journeys with the assistance of information tools such as electronic traffic bulletin-boards.

Council member Bill Burrill noted that Mangere Bridge's Rimu Rd on-ramp was on a downhill slope which gave motorists gravity-assisted acceleration for relatively easy mergers with motorway traffic.

But he foresaw problems at other ramps such as northbound from Mt Wellington to the Southern Motorway, on which climbing trucks may cause holdups if forced by traffic lights to break their momentum.

Mr McCombs acknowledged that merging speeds were important and said his team was looking at making "a reasonable amount" of geometric improvements to motorway ramps as part of its project.

He said Transit might even consider allowing trucks free passage at some ramps with high commercial traffic counts.

A council member who lives in Mangere Bridge, Robyn Hughes, said traffic often banked up outside her house two blocks from the on-ramp and sometimes reached back through the shopping centre.

"It is impacting heavily on the community of Mangere Bridge."

Mr McCombs said efforts would continue to dissuade motorists from "rat-running" through the suburb to reach the motorway, rather than using an on-ramp further south from Coronation Rd.

He said the proportion of vehicles using the Rimu Rd ramp after driving unnecessarily through Mangere Bridge had declined from 30 per cent to 9 per cent, "but it could be better".

He assured committee members that Transit would work with local councils to minimise the impact on suburban streets from widespread ramp-metering, but admitted traffic signal phasing through the agency's computerised network needed considerable improvement.

Transit has taken over the region's traffic management unit from local councils over the years, and Mr McCombs acknowledged a need to "sharpen up our act" to ensure motorists did not have to exceed speed limits to encounter more than one set of green lights.

He described ramp-metering as just one of a range of "travel demand management" measures to influence transport choices, including whether to take a car or public transport, or even whether to set out on a journey in the first place. "It is all directed at changing or influencing individual travel patterns," he said.

"Congestion has a significant drag on the Auckland economy and requires a comprehensive approach - we are shifting Transit from an organisation used to just building bitumen to one that manages roads to best effect."

Mr McCombs said evidence was already emerging of an increase in car occupancy rates, an aim of demand management.

An average rate of 1.15 occupants per car was rising to about 1.3 on the Southern Motorway. Some surveys found it as high as 1.6 in morning peak periods.

Should traffic lights be placed on Auckland's motorway on-ramps?

* For: Supporters say they help vehicles to merge and speed up motorway traffic. Lights at Mangere Bridge have improved speeds on the Southwestern Motorway by 11 to 30 per cent.

* Against: Critics say traffic can bank up. They predict problems with trucks on steep uphill gradients such as the northern on-ramp at Mt Wellington.