Critics question benefits of flyover

The traffic predictions used to justify the Basin Reserve flyover were based on dodgy calculations, critics say.

Retired transport planner John Foster set about discrediting the New Zealand Transport Agency's transport modelling at the flyover's board of inquiry hearing this morning.

Mr Foster, who previously worked on the Transmission Gully highway, Dowse Interchange and Wellington's Inner City Bypass, appeared for flyover opposition group Save the Basin.

In documents presented to the four-member board, he said the benefits of the $90 million project had been overstated, mainly due to errors on the transport agency's part.

One of computer programmes it used to model traffic was flawed because it did not consider a section of road to be a "bottleneck" unless traffic was bumper to bumper for more than two hours.

Mr Foster believed most Wellington bottlenecks were only congested between 70 and 100 minutes. "The model precision is therefore too coarse."

The Basin roundabout was currently at capacity during peak periods but the narrow Mt Victoria Tunnel was preventing that situation from getting any worse, he said.

Hence, the Basin roundabout was not congested, but the tunnel was.

Building a second Mt Victoria tunnel would solve that issue but overload the Basin with vehicles.

However, widening the Basin to three lanes would allow all of that traffic to flow around the cricket ground quite happily, Mr Foster said.

With a flyover in place, state highway traffic would get a smooth ride but local traffic would still be subject to delays at the roundabout's traffic lights.

"Improper base cases have dogged the investigations," he said.

Mr Foster also believed the agency had not accurately predicted how much traffic would flood Wellington's CBD once projects like Transmission Gully and the Kapiti expressway were enticing more people to drive.

"My alternate model-based examination indicates that the intersections along Vivian St will likely be overwhelmed ... thus indicating a material risk of gridlock," he said.

"The resulting situation throughout the Wellington City arterial network could be become intolerable."

Transport agency lawyer Andrew Cameron pointed out Mr Foster's modelling had not been peer-reviewed.

Details of how he came to his conclusions had also been kept from the agency.

The only transport models that had been subjected to a rigorous independent analysis were the transport agency's, Mr Cameron said.

Mr Foster agreed the current scenario at the Basin was not ideal and something needed to be done. But he doubted that a flyover was the answer.

"In my opinion the [flyover] is not reasonably necessary prior to duplication of the Mount Victoria tunnel."