We all know from experience that vehicles don’t always skid evenly when traversing slippery surfaces. But on such surfaces, they may at any point in time be on the cusp of skidding. Indeed, at this point, vehicles are at their most vulnerable to changes in surface conditions which might induce a skid.
These conditions include, but are not limited to:
- Differential skid resistance (friction characteristics) between the cover top profile and its surrounding road surfacing.
- Lubrication of the contact surface by rainwater or contaminants which change the contact friction between tyres and the traversed surfaces. e.g. oil, wet leaves.
- Disruptive cover and/or frame surface edge geometries that have inherently low friction or induce the loss of grip due to unsympathetic alignment to the direction of vehicle travel.
- Thermal effects that change to friction characteristics of the components surfaces in contact. e.g. with variations in tyre elastomers.
- Change in the force applied by the vehicle tyres to the trafficked surface.
- Friction characteristics of the vehicle tyres and their propensity to change contact area with varying inflation pressures.
Of these factors, differential skid resistance between surfaces is arguably the most relevant when selecting a skid resistance requirement for manhole covers used in a particular road surface. In order to achieve accuracy in this aim, their selection should take into account validated skid resistance values obtained from testing at recognised national facilities.
Minimising the differential skid resistance between a manhole cover and its surrounding road surface is vital in reducing the likelihood of a vehicle skid. In support of this, it is also important to recognise that when selecting manhole covers for operation in a given road surface of known skid resistance, its own skid resistance should closely harmonise with that of the surrounding road surfacing. i.e. not be significantly different. e.g. 70 PSRV road surfaces should ideally have 70 PSRV manhole covers installed in them, not those of significantly higher or lower PSRV value.