Friction is defined as the resistance to motion between two surfaces in contact. Its magnitude is expressed by the coefficient of friction (f) which is a ratio of 2 forces, one parallel to the surface of contact between two bodies and opposed to their motion (the friction force) and the other perpendicular to this surface of contact (the normal force).

In the context of road transportation, the surface of contact is the road-tire interface and the normal force is the wheel load. The coefficient of friction ranges from nearly 0 under icy conditions up to above 1.0 under the best surface conditions [70].

Figure 10 Longitudianl and transverse friction

Longitudinal friction concerns the friction on the direction where a vehicle is moving and affects acceleration and deceleration. Transverse friction concerns the available skid resistance at a direction which is perpendicular to the vehicles direction of travel, and allows for change of direction.

The skid resistance of a road pavement is an important road safety factor, especially when the road surface is wet. A concentration of accidents on a wet surface can therefore be an indicator of friction deficiency. The following conditions increase the risk of accidents even more:

• The problem is at a location where the friction requirement is high (e.g. approach to an junction, horizontal curve, downhill slope);
• The problem is isolated (e.g. road surface contamination).

Drivers may have difficulty in recognizing sites with skid resistance problems and as such, they may not reduce their speed at those locations, as would be necessary to maintain their risk at a level they consider acceptable. Several studies have shown that there is a significant correlation between accident risk due to skidding and the pavement's skid resistance. Accident risk due to vehicle skidding on pavements with friction coefficient (SFC) less than 0.45, is 20 times higher than on pavement surface with a SFC higher than 0.60. Moreover, if the SFC of a road is less than 0.30, accident risk is 300 times higher [90].

 The accident risk is higher when the skid resistance is low. Accidents that are related to friction deficiencies occur mostly under wet surface conditions because the available friction is then reduced (PIARC, 2003). These concentrations of wet surface accidents are worst at road locations having both a poor skid resistance and a high friction demand: Page and Butas (1986) found that accident rates on wet pavement were highest in horizontal curves, especially when SFC was less than 0.25. Wet pavement accident rates were also higher for both uphill and downhill slopes (steeper than 3%) than for flatter terrain. Farber et al (1974) report that only 2.3% of wet surface accidents occurred on tangent sections of roads, where the friction demand is low. Viner et al (2005) concludes that amongst the most potentially dangerous driving conditions are those caused by low friction due to heavy rainfall combined with poor road geometry, or those where there is a sudden change in friction, perhaps due to contamination, localized deterioration of the surface or first snowfall.

There exist sufficient studies to indicate that two main characteristics of pavement surface affect skid resistance: microtexture and macrotexture. The role of each in providing sufficient friction varies depending on the speed [60]. However, the most important factor affecting skid resistance is pavement macrotexture, which is the feature that increases skid resistance at high travel speed. Results in Great Britain indicated that accident risk increases when texture depth drops below 0,7mm [93]. Similarly, in a study of crashes and surface characteristics on open roads in France, it was found that wet weather accidents increased markedly for sand patch texture depth less than about 0.5 mm, as shown in Figure 10.

Figure 11 Relationship between Wet-Accident and Surface Texture Depth (Gothie 1991)

Evenness is a measure of the regularity of a road surface. All types of road surfaces (rigid, flexible, gravel, etc.) deteriorate at a rate which varies according to the combined action of several factors, such as the axial load of vehicles, the traffic volumes, the weather conditions, the quality of materials and the construction techniques [70].These deteriorations have an impact on the road surface roughness by causing cracking, deformation or disintegration. Water concentration on these deteriorations increases the risk of vehicles skidding.

When the evenness of a whole road section has sharply deteriorated, users tend to reduce their speed in order to maintain their comfort at an acceptable level, thus minimizing potential safety impacts. Pavement roughness can however be more detrimental to safety when problems are localized, unexpected and significant. Such situations can generate dangerous avoidance manoeuvres, losses of control or mechanical breakdowns of vehicles, thereby increasing the risk of accidents. Reductions in skid resistance caused by vertical oscillations of vehicles on uneven road surfaces can prove problematic, especially for heavy vehicles and when the problems are isolated [70].

 The safety impact of pavement roughness varies according to the type of accident considered [2]: The single-vehicle accident rate decreases as the pavement roughness increases, due to reduced speeds; The multi-vehicle accident rate increases, due to lateral shifts and speed differentials between road users. However, one should also be aware that an improvement in the evenness quality associated with resurfacing may result in speed increases, thereby having a slightly negative safety effect.