Highways Adverse Weather & Winter Service Plan

Extreme Heat

The impact of extreme heat on the highway asset is a relatively new but growing concern. In July 2022 the Met Office reported temperatures over 40⁰C recorded for the first time in the UK as thermometers in Lincolnshire reached 40.3⁰C and 46 weather stations across the UK exceeded the previous UK record of 38.7⁰C. This led to the Met Office issuing its first ever Red Warning for extreme heat.

It is also notable that of the 30 hottest days in the UK by area averages, 14 have occurred this century and the Met Office advise that climate change is making UK heatwaves more frequent, intense and long-lasting. Extreme heat events can have an adverse impact on road surfaces as the dark asphalt absorbs heat through the day and whilst air temperatures may be above 20⁰C, road surface temperatures can exceed 50⁰C.

The development of high temperatures in the asphalt surfacing can cause viscoelastic behaviour, thermal deformation and change the stiffness of the surface. These effects can also cause traffic loading to have a detrimental impact on the surface, such as by causing ‘rutting’ of the surface and, in addition to inducing cracking within the surface, reduce the skid-resistance which has a direct impact on vehicle stopping distances and hence road safety.

During extreme heat events highway surfaces are monitored and areas where surfaces become soft or appear ‘fatty’ or ‘polished’ remedial measures can be taken. Identified sites can be dusted with stone dust or sharp sand which may be dispensed through the gritter fleet. The application of small aggregates helps to restore the skidding resistance of the surface and protect it from direct sunlight as the aggregates settle and are tracked into the surface.

Prolonged periods of high temperature can also have an impact on the underlying subsoils causing materials to shrink and crack as they ‘dry-out’. These cracks can then be reflected up through the pavement to the surface of the road. Damage of this type will require a more invasive intervention to repair.