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13 November 2018 Kate Palmer

Central government and private industry have joined forces in a world-first investigation to assess braking distances on our unique chipseal roads.

The NZ Transport Agency project, which was carried out by WSP Opus and supported by the NZ Police and Upper Hutt City Council, sought to identify whether bitumen tracking that occurs during warm summer months is a safety issue.

Peter Cenek, WSP Opus Project Director, says that when the bitumen binder becomes soft when temperatures are high, it adheres to vehicle tyres and is spread along the road, resulting in the wheelpaths becoming black in colour.

“This causes concern because the tracked material coats the surface of the stone chip creating a slick surface – leading to lowered skid resistance. The NZ Transport Agency wanted to understand the level of safety risk that tracking of soft bitumen of chip-seal road surfaces poses to motorists.”

Following laboratory-based investigations at WSP Opus’ research facility in Petone, a three-day field trial was carried out in the Whitemans Valley/Mangaroa Valley area of Upper Hutt.

John Donbavand, NZTA Lead Advisor Pavements says the results of the testing were a wakeup call.

“We found that the tracked binder was much slipperier than the normal surface in the wet and in the dry. Tests also showed how effective anti-lock braking system (ABS) was in enabling vehicles to stop in a much shorter distance.  Most cars used in New Zealand now have ABS but this is not the case for motorcycles and, from the tests, it was found that motorcycles not using ABS can take twice the distance to stop,” he says.  

Donbavand says this type of investigation using motorcycles has never been carried out before and is a great example of how central and local government, along with the private sector, can work together effectively for the public good.

Both the NZ Transport Agency and NZ Police are committed to improving road user safety and education, particularly for motorbikes.

GoPro and drone footage taken during the field tests will be utilised by the NZ Police and motorcycle training organisations, such as Passmasters, for driver/rider training purposes.


About the testing

Laboratory measurements were conducted at WSP Opus Research to understand how the skid resistance of a medium textured chip-seal surface varies with thickness of the bitumen film.

This information was used to guide bitumen application rates for the field tests.

Field tests included ABS and locked wheel (i.e. non-ABS) braking-to-stop measurements for:

  • Coarse chip seal surface with no bitumen tracking (baseline)
  • Coarse chip seal surface covered with a thick film of bitumen representing a worst case scenario.

Upper Hutt City Council gave approval for testing to be carried out on a 400m section of Gorrie Road in the Whitemans Valley/Mangaroa Valley area of Upper Hutt.

During testing, two vehicles – a police car and a Passmaster’s motorbike - accelerated over a distance of 50m to reach 50kph and, on reaching the test area, “panic” braking was performed to bring the vehicle to a complete stop.

These braking tests were performed under dry and wet conditions with the deceleration during the braking manoeuvre  being continuously logged by the WSP-Opus’ Vericom VC4000 braking performance computer. The braking-to-stop distances and peak and average values of tyre-to-road friction obtained  will be used to improve road surface design and maintenance practices. They  will  also be used to validate modelling used by NZ Police and crash investigators for crash reconstructions.

Skid resistance measurements of the test sections were also made with skid testers routinely used to determine the level of skid resistance of New Zealand state highways. These measurements were compared to the “panic braking” results under wet road conditions to establish if their sensitivity to bitumen film was similar to that of a tyre under severe braking.