Black asphalt road in the field in the winter. Photo: 123RF
Treacherous driving conditions come with the territory at this time of the year, especially in certain regions like the Central North Island and Central Otago where "black ice" is a looming threat.
The problem with black ice is that it's especially dangerous because it's a type of ice you can't see.
It's been reported as the cause of two separate, single-bus crashes that happened 100 metres apart near Tekapo on Thursday.
Free Line Coach Ltd manager Ryan Qiao said three of their buses had been travelling in a convoy for a large group of children, adults and three teachers from China, who had attended the World Choir Games in Auckland last week.
He said the drivers were experienced, but the road "was really bad" with black ice.
It left one person in a serious condition, with two others sustaining moderate injuries.
Black ice is "black" because it's transparent and the road surface shows through.
Meanwhile, ice looks white because there are air bubbles and swirls trapped inside.
Sub-zero conditions, melting snow, fog, gentle rain and even condensation from vehicle exhausts can help black ice to form.
It's most often in calm conditions with little wind.
On a sunny but freezing day, like can be common in Central Otago, heat can melt away the white stuff but what's left refreezes overnight into black ice.
A bus crash involved tourists on State Highway 8 in the Mackenzie District on 18 July, 2024. Photo: Supplied / Grace Duggin
Because it starts as a watery surface, black ice is hard to spot because it's extremely thin. It might look like a harmless puddle.
For that reason, people need to be especially aware of wet-looking patches of road on an otherwise-fine day, especially if they are in an area that doesn't look like it gets much sun.
Bridges are more susceptible to ice of any kind, because cold air can circulate above and below, and there's not as much insulation underneath to stabilise the temperature.
Black ice can hang around much longer on rural roads, because the warmth and friction of traffic wears it away.
What should I do if I hit black ice?
A car can really only do three things: accelerate, steer and brake.
If you're driving on ice and/or snow with very little traction, the key is to only do one of those things at a time.
But the problem with black ice is that by the time you've hit it, panic might have already taken over.
The difference between white ice and black ice is that the former is more likely to be sustained. This means you'll be aware you're driving on it, it'll go on for a long distance, and you can modify your driving style to suit.
With black ice, there will be a clear beginning and a clear end - more than likely after a few metres.
So if you hit black ice and lose control, don't panic and try to fight your way back - because you won't be able to. Your car will have virtually no grip or traction.
It's best to remain calm, try and keep the steering wheel at the straight-ahead, and wait for the black slippery stuff to pass.
- Stuff