One trucking company has limited its speed to 80kmh while on the alternate highway through the upper South Island due to safety concerns (file photo).

Safety concerns and a near miss have forced one trucking company to reduce drivers speed limits on the South Island's challenging alternative highway, but its not likely to become required by all companies.

Transport company Carr and Haslam has restricted drivers speed limits to 80kmh while on the main highway through the upper South Island, down from the 90kmh truck limit. It is also keeping trucks off the road from about 10pm to 4am.

Since the November 14 earthquake closed State Highway 1, South Island traffic has been forced to take an alternative route over the Lewis Pass when moving between Christchurch and Picton.

FAIRFAX NZ There has been a huge increase in heavy traffic on the alternate highway since the November earthquake.

The highway is a longer and more challenging drive than the route it replaces, and is a road never designed to handle such large volumes of traffic. Since the quake, traffic on the route has increased dramatically with the average number of heavy vehicles passing through the Lewis Pass rising from 207 to 787 daily.

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* Lower speeds on alternate route

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* Christchurch pair killed in crash on 'challenging' alternate highway as lower speeds mulled

Carr and Haslam managing director Chris Carr said the reduced speed limits were put in place about four or five weeks ago, following an incident one dark night when one of their vehicles collided with a truck travelling travelling the other way.

Carr said there was a "glancing blow to our trailer by the other truck, which caused our driver to lose control". His driver claims the other truck was in her lane.

The truck swung back and forth and jack-knifed before the driver, Brenda, was able to bring the vehicle under control.

Carr said the incident happened within four seconds. He said Brenda "reacted incredibly well", but there was a degree of luck involved.

"If she'd been two seconds slower, then maybe she wouldn't have been in that place. Who knows?"

He said by reducing the speed limit his drivers were travelling at, it "gives the driver that few extra seconds in the case of anything goes wrong".

The changes made scheduling more difficult, but it was "not as difficult as it would be picking up a broken truck . . . or damaged cargo on the side of the road".

"I don't want our staff put under any more risk than they have to."

The speed limit on many sections of the route have already been reduced to 80kmh in response to the extra traffic.

Carr said lowering speeds would also reduce damage to the road, which would be crucial for the South Island over winter.

"The good people of Christchurch have no idea how tenuous their supply of goods is. Everything that they get, is just hanging on that one piece of road."

Road Transport Forum chief executive Dennis Robertson said he thought the speed reductions were "very sensible given the fact that we're entering into the winter conditions", but he did not want every trucking company to have to do it.

Only slowing down trucks could create a safety issue if people tried to "overtake when they shouldn't", he said.

North Canterbury Transport Infrastructure Recovery Alliance journey manager Tresca Forrester said there was no legislation that would enable the NZ Transport Agency to change the rules for trucks on just one section of the state highway system.

Carr did not think every trucking company should have to reduce their speeds.

"It's entirely over to them what they do."