Tourists - including singers in NZ for the World Choir Games - were injured in two separate crashes 100 metres apart on the Tekapo-Twizel Highway on 18 July. Photo: Supplied / Grace Duggin
Temporary lower speed limits are being considered for parts of a treacherous highway in the Mackenzie District, after a spate of serious crashes.
Twenty-four people have been injured in crashes on State Highway 8 near Lake Pūkaki in the space of a week.
Nine people were injured in a multi-vehicle crash on Sunday - just days after two buses carrying international tourists rolled in separate crashes about 100 metres apart, leaving 15 people in hospital.
Travellers and locals described the conditions as hazardous, with black ice and low visibility.
New Zealand Transport Agency (NZTA) central South Island systems manager Mark Pinner said the high number of crashes in this patch was unusual.
"It's caught us on the hop a little bit because historically, year to year, we haven't had problems.
"There will always be a few accidents here and there, but none like the spate we've just had and none to the severity we've just had," he said.
"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 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 State Highway 8'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 21 July] 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 NZTA for a briefing on what information they had and what information they were providing motorists at the time of the recent State Highway 8 crashes.
"NZTA will be conducting a full investigation alongside police to ascertain what the contributing factors were for these crashes. If the investigations finds 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, 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 that Roadside Variable Messaging Signs (VMS) had been activated.