A lethal combination of worn-out roads, worn and wrongly inflated tyres is putting lives at risk, according to a new report from the AA Motoring Trust and the County Surveyors' Society (CSS)[1]. 17 per cent of main roads fail basic skid resistance tests, one in 10 cars are running on illegal (under 1.6mm tread depth) tyres, around nine in 10 tyres are incorrectly inflated and an AA Trust survey has found up to half forecourt air pumps to be inaccurate.
"Tyres are critical car safety components; they are the easiest for motorists to check, yet most don't. But road surfaces are critical too and road authorities must have, and fully deploy, funds to provide safer surfaces. Doubling skid resistance can halve the number of accidents, while driving on worn-out roads on worn-out tyres is courting disaster. The quality of surface maintenance on many roads is not good enough and the minimum threshold for tyre tread depth may no longer be adequate. A review is now needed of the minimum legal tyre tread depth of 1.6mm based on research evidence from crashes," says Bert Morris, the AA Trust's Director.
These are key recommendations to emerge from GET A GRIP - Tyres, Road Surfaces and Traffic Accidents (PDF 817K)fromthe AA Trust and the CSS. The result ofa two-year study[2], the report is the first to probe the combined safety implications relating to the quality of the tyres and the roads on which we drive.
Geoff Allister, CSS President says, "How well rubber and road grip each other can be the difference between life and death in crashes and near misses. We need much greater investment from local authorities in road surface renewal – few other budgets have such an influence on life and death in their communities. It's time for society to treat road crashes resulting in death or severe injury as seriously as those in the air, on the railways or in the workplace."
The research reveals:
- Motorists' alarming neglect of the condition of their tyres;
- A significant variation in the quality of road surfaces;
- Damaged and faulty tyre pressure kit – an AA Trust survey (June 2005) found almost half of forecourt air pumps to be inaccurate; and
- Major gaps in systems and knowledge about how best to prevent death and serious injury due to tyre and road wear.
GET A GRIP has messages for everyone responsible for tyre and road surface safety:
Government
Review the legal minimum of 1.6mm for tyre tread depth, based on research evidence from crashes.
Road authorities
Do much more to meet the legal duty to maintain road surfaces.
- The likelihood of skidding accidents increases by 50 per cent on busy sites where road surfaces get excessive wear (about 10 per cent of the road network) and fall below recommended thresholds.
- Achieve better skid resistance indicators that link to the risk of accidents by redesigning the annual National Road Maintenance Condition Survey.
Motorists
Check tyres and air pressure weekly (preferably when tyres are cold) – it can save lives and money. Be aware that tyre pressure equipment may be faulty.
- Monitor tyres more closely once they have reached 3mm: replace them as soon as tread depth drops to 2mm.
- Reduce speed when it is wet: even new tyres cannot corner as fast in the wet as in the dry, especially on sub-standard roads.
Garages
Keep pressure gauges easy to use, accurate and clean or remove from service.
Manufacturers
Develop affordable on-board tyre pressure monitoring for all cars.
Do more research into:
- How their tyres performed in actual, not simulated, accidents; and
- The effect of differing front and rear wheel tread depths in causing crashes.
Police
Following every fatal road crash, incorporate in national accident records (STATS 19) accurate information about road surface, as well as tyre type and condition, to assess how they may have been contributory factors.
Trading standards officers
Check accuracy of tyre pressure equipment more often to ensure legal compliance.
Summary of findings
Tyres
Worn tyres
- Ten per cent of cars have one or more tyres with a tread depth at or below the legal (1.6mm) limit. Research more than 30 years ago at the TRL (Transport Research Laboratory) produced broadly similar results.
- Worn tyres reduce braking capability in the wet and severely increase the risk of accidents.
- In wet conditions, tyres with less than 1mm tread depth provide one-third the braking friction of tyres with the minimum legal tread depth of 1.6mm.
In the wet:
- Stopping distances are doubled;
- Worn tyres contribute to about one in 10 accidents compared to one in 50 accidents on dry roads;
- Grip is markedly reduced when tread depth is less than 3mm; and
- The risk of an accident is three times greater than on dry surfaces, if the tread depth is at the 1.6mm legal minimum; it increases seven-fold when the tread depth is less than 0.5mm.
Tyre imbalance
- Different tread depths on front and rear tyres create handling problems.
- Vehicle handling is affected when tread depth is substantially greater on the front tyres than on the rear tyres.
- When replacing two tyres at the same time, fit them to the rear wheels.
Wrong air pressure
- Surveys of the accuracy of air pressure gauges at garages have shown one in five to be defective; and an AA Trust survey (June 2005) found almost half those workingto be inaccurate.
- Surveys have shown up to nine in 10 tyres to be incorrectly inflated.
- Three in four tyres are under-inflated.
- Under inflation increases tyre wear and fuel consumption; it can also cause vehicle-handling problems.
- Tyre life drops by up to 10 per cent for every 10 per cent of under inflation.
- Over-inflated tyres wear unevenly and can compromise braking friction, as less tyre is in contact with the road.
Structural defects in tyres
- Tyre quality and performance have improved in recent years.
- Structural failures in tyres are responsible for less than four per cent of accidents. These are usually caused by a tyre running for an extended period at low pressure or suffering damage before the accident occurred.
Accident risk
- Research from Finland shows that on slippery roads, at speeds above 40mph, the car that lost control and crashed was six times more likely to have worn tyres than other cars involved.
- Australian research shows that drivers of vehicles with worn tyres may have other characteristics that increase accident risk – eg, young (61 per cent were male and in their early teens and early 20s), driving older cars and paying higher insurance premiums.
Roads
Surface texture and skid resistance
- It is surface texture that creates skid resistance.
- Road surfaces have a macro-texture (overall roughness resulting from the number, type and size of the stone chippings) and a micro-texture (roughness of the individual chippings).
- Accident rates increase markedly as the surface texture elements wear and get smoother, and surface friction decreases.
- There is a skid resistance standard for national roads and code of practice for local authority roads. They provide guidance on how to assess the quality of the road surface and how to decide when it is necessary to improve the surface.
- On national roads, the standard relates the level of wet friction of the road surface to the risk of wet skidding accidents. On sites found to be below the specified threshold, there is a more detailed examination of surface condition, accident history and the risks linked to the nature of the site. This procedure is robust if implemented correctly.
- Local authorities have a maintenance code of practice for their roads but they vary more in the extent to which they implement a standard.
- Little is known about the link between skid resistance and accidents on typical local road geometries and layouts.
- Some surfaces have less skid resistance shortly after being laid than they do once traffic has used them for some time. They can be as slippery in the dry as they are in the wet for the first weeks, and possibly months. However, the means to measure dry-skid resistance of new-laid asphalt is not yet widely available. This needs urgent investigation.
The National Road Maintenance Condition Survey (NRMCS)
- A skid resistance indicator in the annual National Road Maintenance Condition Survey, which tracks the overall condition of the country's roads provides onlya single statistic for the overall skid resistance condition of main (principal) roads.
- This single statistic does not relate directly to the risk of an accident occurring, but does provide a first-level safety indicator.
- About one in six of all sites on principal roads fail first-level safety checks.
- High-stress sites (such as roundabouts, sharp bends and steep hills where there is a lot of braking) make up about 10 per cent of the total network.
- Two thirds of such sites fail first-level investigation checks, and they are almost 50 per cent more likely to have wet-skid accidents than sites that pass the check.
- Low-risk sites (those where there is little wear caused by braking) make up a substantial proportion of the national road network. Sites that fail the first-level safety check have a five to 10 per cent greater risk of a wet-skid accident compared with sites that pass.
New surface designs
- "Negative texture" new generation surfaces reduce tyre noise and spray.
- Low-noise surfaces are popular with motorists and residents. Government proposes that 60 per cent of national roads should be low-noise by 2011.
Retexturing:
- Usually means 'scarifying' or roughening the existing surface;
- Can effectively restore skid resistance and texture depth in the short- to medium-term to a sound road surface with poor micro- and macro- texture;
- Can provide quicker surface treatment than conventional resurfacing techniques;
- Costs may be only 10 per cent of those for resurfacing;
- Is environmentally friendly, partly because it does not require new materials such as scarce premium aggregates; but
- May be only a short-term solution on highly trafficked or stressed sites.
High friction surfacing:
- Is extremely effective in reducing accidents at high-risk sites, such as approaching junctions and zebra crossings;
- Can reduce accidents by half on bends;
- Can now be specified through an approval scheme through the British Board of Agrment Product Approval Scheme; but
- Is not a panacea for all high-stress sites. Among considerations, treatment has to be matched to how long the product is likely to last at the site.
Spray, weather, risk and databases
- New-generation road surfaces can reduce road spray.
- Current regulations (set in 1984 and 1991) limit measures to improve the suppression of spray from vehicles – the need for easy-to-produce designs for mass markets means that the regulations stifle potential innovation.
- These regulations do not relate closely enough to how much spray is reduced to increase visibility for following drivers.
- Those exempt from UK regulations restricting spray include vehicles from other countries, those under 12 tonnes, as well as bulk tankers and tippers.
- The skid resistance of a road surface varies according to recent weather and the levels of detritus on the road.
- Less rainfall in summer generally means that the surface friction of a road is lower compared with the winter, although ice also causes skidding.
Risk
Running on worn surfaces greatly increases accident risk, especially in the wet, the key road-tyre factors determining accident risk include:
- Tyre tread depth;
- Vehicle speed;
- Road surface texture and skid resistance;
- Road shape and geometry;
- Properties of the tyre rubber and how it performs when hot;
- Temperature and deformation of the tyre;
- How much of the tyre is firmly in contact with the road;
- The extent to which water is forced from the contact patch between the tyres and the road; and
- The presence of local contaminants on the road, such as oil or detritus.
Databases
- STATS19 (T1 in Northern Ireland) is the legally required record of road accidents involving personal injury. It is a valuable and ever improving database of information on personal injury road accidents in the UK.
- STATS19 provides data, recorded by the police after the event, about the general layout of the accident location, the time and weather, and the vehicles and casualties involved.
- STATS19 lacks detailed information on the nature of the tyre-road interface.
- Better availability of data on contributory factors in accidents is providing vital new information about vehicle defects and environmental conditions.
- MOLASSES (Monitoring Of Local Authority Safety Schemes)is a database containing information about road safety schemes implemented by local authorities in the UK.
- MOLASSES details more than 4500 schemes installed since the late 1970s.
- Since the mid-1990s, submissions from local authorities of new schemes for the MOLASSES database have dropped.
- MOLASSES is not always easy to use and needs revitalising.
Download
"GET A GRIP Tyres, road surfaces and reducing accidents: a review" (PDF 823K)
Related Links
Tyre Industry Council:
http://www.tyresafety.co.uk/
National Tyre Distributors Association (NTDA):
http://www.ntda.co.uk/
The Highway Authorities Product Approval Scheme (HAPAS): http://www.bbacerts.co.uk/hapas.html
The Institution of Highways and Transportation:
http://www.iht.org.uk/
British Board of Agrment BBA:
http://www.bbacerts.co.uk/frames.html
Refined Bitumen Association:
http://www.bitumenuk.com/
Road Emulsion Association Limited:
http://www.rea.org.uk/
RoSPA: Tyre Information Sheet
[1] CSS represents chief officers in local highway authorities.
[2] Bullas, John C. (2004) Tyres, road surfaces and reducing accidents: a review.
