Can the UK’s roads withstand extreme heat

Dr Ian Lancaster, Technical Director, Asphalt Industry Alliance (AIA), explains what happens to asphalt when temperatures rise and why roads can appear to ‘melt’ in the heat.

The Met Office has reported that the UK recorded more days with temperatures of 30 degrees or above in 2026 than it did during the whole of 1976 – a year remembered for its record-breaking summer heatwave.

What was once regarded as extreme weather is increasingly being considered as normal. And, with more hot days comes an issue that is reported every summer: that under certain conditions, the surface of the road can appear to ‘melt’, raising the question of whether roads in the UK are built to withstand the heat.

Asphalt has been used for many decades to build the majority of roads in the UK, as well as footpaths, car parks and airport runways, but now has to be able to cope with road temperature extremes which can range from +60ºC to -20ºC. This is a significant shift in the challenge for design engineers more used to evaluating performance in a narrower range, and who also now have to take the impact of more extreme rainfall into consideration.

Why roads appear to ‘melt’

Roads absorb heat, particularly in direct sunlight, and reach higher temperatures than the air around it. In extremely high temperatures the road surface doesn’t ‘melt’ but the bitumen in it can tend to soften, leading to the road surface particles potentially becoming more ‘mobile’.

Bitumen is a complex material, and its viscoelastic properties mean it can show both elastic (spring like) and viscous (oil like) behaviour depending on the temperature and traffic loading. As temperature decreases it has a more brittle behaviour and as temperature increases it behaves more like a thick liquid. The same is true for traffic loading and speed. At high speed and low traffic load, bitumen behaves in a more elastic fashion, while under heavy, low speed loads it behaves more like a viscous liquid.

The effect is most exaggerated on south-facing gradients but is usually limited to the top surface and doesn’t generally affect the structural integrity of the road after heatwaves abate. However, in prolonged periods of hot weather where the road is exposed to slow moving or stationary heavy traffic, the risk of road particle migration and permanent deformation of the surface increases – especially on poorly maintained roads with existing defects.

Authorities have reactively treated ‘melting roads’ by applying fine crushed stone grit which acts to both reduce heat absorption of the surface (being a lighter colour) and to ‘stabilise’ the softened or mobile material.

Building resilience through material innovation

Ongoing research in modern road building materials is continuing to build both resilience and flexibility into the network, improving their performance across a wider temperature range. For example, the development of polymer modified bitumen (PMB) products that improve temperature resilience to increase durability and reduce the number of maintenance interventions. In addition, industry suppliers are conducting ongoing research into enhancing resilience per se to help futureproof the network.

But in a climate of increasing costs, as well as temperatures, and against a backdrop of constrained or reducing maintenance budgets, highway authorities face even more challenges to future-proof their assets. It is therefore essential that local authorities are given the resources they need to proactively ensure their roads are climate resilient.