Melted roads caused by hot weather pose a potential safety hazard this summer

Friday, 23 November 2018

The NZ Government and private industries have joined forces in an investigation to assess braking distances on our chip-seal surfaced roads. (Video first published in November 2018)

With summer just around the corner and 2018 already shaping as one of the deadliest for New Zealand motorists in recent years, the magnitude of just how dangerous melted roads are has been revealed.

Recent testing by WSP Opus on chipseal roads affected by bitumen tracking – where melted bitumen makes the road surface slicker – found vehicles with anti-lock braking systems (ABS) took 30 per cent longer to stop when the brakes were applied, while vehicles without this safety feature skidded for double the distance they would on normal chipseal.

New Zealand is one of only a few countries to predominantly use chipseal on its roads. About 68 per cent – or 64,000 kilometres – of all roads in this country are sealed with it.

Bitumen tracking occurs whens softened bitumen is collected on vehicle tyres and spread along the road, creating black tracks.

Laurence Harrow, technical specialist in transport surfacings at WSP Opus, said bitumen generally began to soften when the road temperature reached upwards of 45 degrees Celsius. For that to happen, the air temperature only needed to be about 27C or hotter.

Concerns over roads deteriorating faster than expected prompted NZTA to overhaul its bitumen standards in 2016.

And with climate change expected to increase the number of days capable of melting roads in this country, road safety campaigners are calling for action now to safeguard motorists.

Clive Matthew-Wilson, editor of the car review website Dog and Lemon, said the problem was two-fold.

'On uneven road surfaces, it may take far longer to come to a halt in an emergency.

'Secondly, these uneven surfaces may cause a vehicle to handle poorly. This is particularly serious for bikes or motorbikes because an unstable road surface may cause the rider to lose control and fall into the path of oncoming vehicles.'

This image shows the stopping distance of a vehicle with an anti-lock breaking system (ABS) compared to one without ABS on a road affected by bitumen tracking.

Matthew-Wilson also questioned whether some roads were being built too quickly and too cheaply.

'It's possible to build roads that give few problems, but these cost more. However, even the more expensive roads don't appear to be very well-built.'

The road surface in the Wairau Valley, west of Blenheim, reached 60 degrees Celsius last summer, causing the bitumen to melt. (File photo)

In response to the test results, the New Zealand Transport Agency (NZTA) says it is developing an 'improved specification for bitumen with climate change in mind', and trialling harder road surface binders that are more resistant to tracking.

Sweltering heat last summer caused roads to melt in Marlborough, Wairarapa, Canterbury and Manawatū, where the softened bitumen collected on vehicle tyres and was spread along the road, resulting in black tracks appearing.

A police car hits the brakes on a road in Upper Hutt affected by bitumen tracking to test the road's skid resistance.

WSP Opus road surface characteristics principal researcher Peter Cenek said bitumen tracking 'masked the harshness' of the chipped surface, resulting in less grip on the road.

Caroline Perry, New Zealand director of road safety charity Brake, said the findings were 'concerning' and they would like to see more investigation into how this problem could be reduced.

Hot weather softens the bitumen used to bind chipseal roads. Vehicles pick up the bitumen on there wheels and spread it across the top of the road making the surface more slick. Bitumen tracking on State Highway 63 last summer.

Professor James Renwick, a climate scientist at Victoria University, said the country was experiencing a higher number of 'hot days' of 25C or above as a result of climate change.

He pointed to a Royal Society of New Zealand Te Apārangi report that showed the average number of hot days a year for most parts of the country was expected to rise from 20 to 40 days at present to between 60 and 100 by 2100.

A motorbike hits the brakes on a road in Upper Hutt affected by bitumen tracking to test the road's skid resistance.

NZTA road safety director Harry Wilson said chipseal was used in this country because it was suited to the 'flexible nature' of many New Zealand roads.

'Chipseal is the most common type of road surface used because it [is] durable, hard-wearing and cost-effective, and performs the same, if not better, in some situations [than asphalt].'

Tracking occurs on the open road and in urban areas such as on Cambridge Terrace in suburban Lower Hutt.

Bitumen tracking had not been linked to an increase in crashes, he said.

'Tracking is a very specific phenomenon that only occurs in specific conditions on certain pavements. It tends to only last two to four days.'

John Donbavand, NZTA's lead advisor on pavements, said roads with high traffic volumes, sharp bends or a lot of turning traffic were sealed in asphalt. The rest were chipsealed, which was typically five times cheaper than asphalt.

Automobile Association spokesman Dylan Thomsen said it was pleasing to see NZTA carrying out research on bitumen tracking, and was hopeful the findings would help make roads safer.

As of Friday, the year-to-date road toll stood at 337 – six more deaths than at the same point last year, which turned out to be the deadliest year since 2009. A total of 378 people died on New Zealand roads in 2017.