“I think it’s recognised locally that it’s been a probably different kind of winter than we’ve had in more recent times ...
“The colder weather has lingered more often, we’ve had more frost ... [and] freezing fog does cause problems.”
Pinner said the NZTA was considering whether measures such as applying ice grit and calcium magnesium acetate (CMA) to the road were enough.
It was looking at reducing SH8′s speed limit of 100km/h to 30km/h or 50km/h in some parts.
“It would be hotspots where we’ve generally put more grit or CMA over the last few years, where we know there’s possibly greater risk, so that would be bridges, high-speed curves, possibly some of the hills, and a few places particularly where it’s shaded – certainly the dampness is part of the problem,” Pinner said.
It was sometimes difficult to judge if a road ought to be closed because of black ice, he said.
“It’s easier when there’s something more physical on the road – you see snow, and you can understand that needs to be ploughed and got rid of until vehicles can get through again.
“Black ice is very hard to judge when it is forming and hard to know when it will clear itself.”
Pinner urged people to drive to the conditions and take care on icy roads.
“People need to be aware even if it’s a nice sunny day, there could still be ice.
“The incident [on July 21] happened at 10.30am in the morning and we’d been through there and gritted it just 10 minutes before, and ice still formed.
“So we need people to still be cautious and ... the lower speed limit will hopefully just give people an indication that that’s a risk ahead.”
Transport Minister Simeon Brown said he had asked the NZTA for a briefing on what information it had and what information it was giving motorists at the time of the recent SH8 crashes.
“NZTA will be conducting a full investigation alongside police to ascertain what the contributing factors were for these crashes. If the investigations find that the agency needs to make changes to their operating procedures, then they will do so,” he said.
In a statement about Thursday’s bus crashes, the NZTA said it would be investigating the road conditions at the time and the condition of the buses.
NZTA said it had issued an area warning about black ice and winter driving conditions the day before, and roadside variable messaging signs had been activated.