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Slippery when wet and dry

YOU'RE DRIVING along a bumpy road, or stuck in miserable roadwork delays.

Then comes an oasis like stretch of smooth, new blacktop surface, the national speed limit applies, and the open road beckons.

Most would put their foot down. But if you are unfortunate enough to have to put your foot down on the brakes, the results could be as surprising as the results of new studies by transport research body TRL.

Newly laid stone mastic asphalt (SMA) roads in dry conditions are as slippery as wet roads, according to the TRL. And other newly laid asphaltic surfaces could behave in the same way, it concludes.

The phenomenon has never before been studied, because all conventional road skid resistance testing by engineers has been carried out in wet conditions - based on a view that this presents a worst case scenario. The new research reveals that this may not necessarily be the case.

How the study came about was literally by accident.

Derbyshire police officers investigating two fatal crashes on new SMA roads carried out routine skid tests. Police accident investigators, unlike engineers, test in dry as well as wet conditions to measure the road friction.

'We skidded a vehicle in the dry on stretches of road between a year and 18 months old, and found that the friction appeared to be quite low. While this was not a contributory factor to the accidents, we had identified a problem, ' said Derbyshire police traffic support unit constable Jim Allen.

The police alerted Derbyshire County Council engineers who commissioned TRL to investigate further. The fact that current Highways Agency tests are not carried out in the dry is not a cause for criticism says the author of the TRL research.

'The police are not assessing the same thing as road engineers. They are trying to find out the friction between the road and locking wheels in an accident. Road engineers are much more interested in the underlying condition of the road rather than its condition at a particular point in time, ' says TRL senior research scientist Peter Roe.

The study focused on SMA, a surfacing material which has become more popular recently in the UK. Unlike hot rolled asphalt (HRA) where stone chips are added in-situ to the hot tar, with SMA, the aggregates are pre-mixed with the binder, and so can be laid in a wider range of weather conditions.

As well as providing good spray performance, SMA provides an extremely quiet surface. Coupled with the Highways Agency's move away from concrete surfacing, the invogue material is being widely used. Originating in Germany, it was first used in the UK around 10 years ago, some time after the Highways Agency's current skid resistance standards were drafted. Its widespread use means that its skid resistance properties need to be fully understood.

'The Highways Agency is reviewing its skidding standards which have been around since 1988. Some things need to be more clearly explained, such as the early life performance of some road surfaces, ' says Roe. Dry skid tests are likely to be on the agenda, given the results of the Derbyshire studies.

Roe's tests included standard Scrim and Skidman tests with Derbyshire police which involved skidding cars in the dry. A Highways Agency machine known as a Pavement Friction Tester (PFT) was also used. Based on a US model, it is the only one in the UK and allows a variety of skid tests using a trailer with wheels which can lock.

Computerised monitoring takes readings at 0.01 second intervals to determine peak and locked wheel friction.

The tests showed that skid resistance in dry conditions over the SMA surfacing was similar to the results for wet conditions. In wet conditions, reduced friction comes from water breaking the high pressure bond between tyre and road surface, the report explains.

However, it says that the excess bitumen found on the surface of early life SMA is subject to high shear stresses in a skid, weakening its bond with the rest of the binder film.

Heat in the tyre tends to reduce the binder viscosity and help it to flow, leading to the reduction in road friction.

Test results using the PFT showed a co-efficient of friction of 0.77 for a non-SMA stretch of road in the dry, around normal. But the friction in dry conditions along an SMA stretch was found to be only 0.45 - 42% less - and similar to the values found for the wet roads tested.

The behaviour of motorists on new roads is not examined, but it is likely that they expect a high quality surface.

'There are suggestions that some people may tend to drive faster on new roads, but there is no objective study of this.

Others may see a shiny road and drive slower, ' says Roe.

Nor is there any clue as to whether drivers make subliminal judgments on braking distances, thinking the new road has a 'normal' high skid resistance, rather than the unexpectedly lower values that the Derbyshire studies have uncovered.

While the report deals with the specific set of circumstances of emergency braking causing wheel locking on a newly laid SMA road, the condition could have wider implications. The report warns that the effect may apply to 'many newly laid asphalt surfacings'.

The report raises other questions. 'Further work is necessary to determine how long this condition can last in order to decide on an appropriate strategy, either to reduce the risk or to warn motorists for the time that the problem exists.'

The further work has already begun, with the Highways Agency skid review examining the report's findings. Further tests on other materials and using cars with anti-locking brake systems are believed to be on the agenda.

Diarmaid Fleming

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