Driving in hot weather: UK roads and your car
Extreme heat does more to UK roads and vehicles than most drivers expect. Tarmac can soften under heavy traffic, cars can break down in far greater numbers, and train cancellations can push more people onto already-congested roads. Knowing what heat does to your tyres, your engine, and the roads themselves can be the difference between arriving smoothly and sitting on the hard shoulder.
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Does tarmac actually melt in UK heatwaves?
Not quite "melt" - but it does soften. UK road surfaces are made from bitumen, which starts to soften when temperatures reach around 50 degrees Celsius. The air temperature in a heatwave might be 35 to 38 degrees, but road surfaces can reach 65 to 70 degrees in direct sunlight - well above the softening threshold.
What you see is not a liquid puddle but rutting: wheel tracks forming in the surface, especially at junctions and on steeper roads where vehicles are braking and accelerating. Sticky patches are also common. National Highways applies specialist anti-rutting treatments to the busiest routes before summer, but quieter A and B roads often do not receive the same treatment. If you spot fresh surfacing that looks soft or shiny on a hot day, slow down and allow extra stopping distance.
Do roads and bridges close in extreme heat?
Road closures caused solely by heat are uncommon, but they do happen. Bridges are more vulnerable than standard road surfaces because steel expands significantly in high temperatures. The expansion joints built into bridge structures have limits, and in extreme cases a bridge may face a reduced speed limit or a temporary closure for inspection.
The Dartford Crossing is one that attracts attention - better known for wind-speed restrictions, very high temperatures can also trigger precautionary measures. Railway bridges carry an additional constraint: track buckling becomes a risk above 36 degrees Celsius, which is why rail services slow down or cancel on the hottest days. Before a long motorway trip on a peak heatwave day, it is worth checking National Highways travel alerts for any live restrictions.
Why cars break down more in hot weather
The AA and RAC both report significant spikes in call-outs on the hottest days of the year. Most heat-related breakdowns fall into three categories: overheating cooling systems, tyre failures, and flat batteries.
The cooling system works hardest in slow-moving traffic on a hot day - exactly the conditions a heatwave produces. Low coolant, a worn radiator hose, or a failing water pump can all push the temperature gauge into the red. Air conditioning adds extra load on the engine at the same time. Batteries also degrade faster in heat than in cold, so an older battery that survived winter may give up in a summer traffic jam. If your temperature warning light comes on, our guide to car dashboard warning lights explains what it means and what to do immediately.
What happens to your tyres in the heat
Tyre pressure rises as the air inside expands. A tyre that was correctly inflated on a cool morning can be noticeably over-inflated by midday on a 35-degree day. Over-inflated tyres have a smaller contact patch with the road, which reduces grip - the opposite of what you need on softened, rutted tarmac.
They are also more prone to blowouts if you hit a pothole or a sharp kerb edge. Check your tyre pressures when the tyres are cold, before the car has moved that day rather than at a petrol station after an hour on the motorway. If you are unsure what pressure your tyres should be, the correct figure is in your car's handbook or on a sticker inside the driver's door. Do not let air out of a warm tyre to bring the pressure down - it will drop too far once the tyre cools again.
Flash floods after a heatwave
The thunderstorms that often follow a prolonged period of heat can be sudden and intense. Parched, sun-baked ground cannot absorb water quickly, so surface water builds up far faster than it would after rain in a normal wet spell. Underpasses, dips in the road, and low-lying routes near rivers can flood within minutes.
Never drive through standing water if you cannot judge its depth. Thirty centimetres is enough to stall most cars; sixty centimetres will float a small vehicle. Cars swept off flooded roads are a real event in UK summers, not a theoretical risk. If a road ahead looks flooded, turn around if / when safe to do so and find an alternative route. For more on handling wet-road conditions safely, our guide to driving in bad weather covers aquaplaning and wet-weather stopping distances.
More traffic when trains and tubes fail in the heat
Hot weather makes public transport unreliable. Rail tracks can buckle above 36 degrees Celsius, triggering speed restrictions or cancellations across the network. Several London Underground lines have no air conditioning and become unsafe to operate at high enough ambient temperatures. When trains and tubes cancel or cut services, drivers fill the gap - on days when more people are also heading to the coast, parks, or countryside.
The result is motorways and A roads significantly busier than a normal weekday, sometimes at short notice. Budget extra time for any journey on a peak heatwave day, especially during school holidays. Our guide to motorway driving rules covers how to handle high-traffic conditions and lane discipline when roads are slow.
Before you drive in a heatwave
A few quick checks before you set off take less than five minutes and can prevent a breakdown:
- Coolant level: check when the engine is cold. The reservoir is usually a translucent plastic bottle near the radiator with min and max marks on the side.
- Tyre pressure: check when cool, before the car has moved that day.
- Water in the car: dehydration slows reaction times measurably. Carry at least a litre.
- Screen wash: heat drives more insects onto windscreens. Visibility on a summer motorway depends on a clean screen.
- Air conditioning: if the AC blows warm air, it may need a regas - worth sorting before a long summer drive.
- Planned route: check National Highways and your mapping app for traffic alerts. Heatwave congestion can make alternative routes faster.
Staying safe while driving in extreme heat
Dehydration has a real effect on driving ability. Research has shown that a mildly dehydrated driver makes a significantly higher number of errors. Drink water before you set off and keep more within reach in the car. If you feel lightheaded or excessively warm while driving, pull over somewhere safe, get out of the car, and cool down before continuing.
Sun glare is most severe in early morning and late afternoon on summer days. Keep your windscreen clean - a smeared or greasy screen becomes almost opaque in low sun - and make sure your sun visor is intact and functional. Polarised sunglasses help significantly with glare from wet road surfaces after a passing shower.
Never leave children or animals in a parked car. Even with windows partially open, the interior of a car in direct sunlight can reach 50 degrees Celsius in under 20 minutes. This is dangerous for adults, and potentially fatal for children and pets.
If a breakdown puts your own car off the road and you need to borrow or use another vehicle, temporary car insurance lets you add cover on a car you do not own from one hour upwards - without affecting the owner's annual policy or no claims record.
Frequently asked questions
Does tarmac melt on UK roads in hot weather?
Not exactly - but it softens. Road surfaces can reach 65 to 70 degrees Celsius in direct sunlight on a hot day, which is above the roughly 50-degree softening point of bitumen. The result is rutting and sticky patches, particularly at junctions and on steeper roads.
Can bridges and roads close because of hot weather in the UK?
Yes. Steel bridges expand in extreme heat and may face reduced speed limits or temporary closures as a precaution. Rail bridges carry additional risk because track buckling starts above 36 degrees Celsius, which is also why rail services slow down or cancel on the hottest days.
Why do cars break down more in hot weather?
Overheating cooling systems, tyre blowouts, and flat batteries account for most heat-related breakdowns. The cooling system works hardest in slow-moving traffic on hot days, batteries degrade faster in heat than in cold, and tyre pressure rises with temperature, increasing blowout risk.
Should I check my tyre pressure before driving in the heat?
Yes, and check it when the tyres are cold before the car has been driven that day. Pressure rises in heat as the air inside expands, and over-inflated tyres have reduced grip and a higher chance of blowing out on potholed or rutted summer roads.
What should I do if there is a flash flood on the road after a heatwave?
Turn around if / when safe to do so and find an alternative route. Never drive through standing water you cannot judge the depth of. Thirty centimetres is enough to stall most cars, and sixty centimetres can float a small vehicle. Parched ground after a heatwave absorbs water very slowly, so floods can form in minutes.
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