Study finds traffic heat measurably raises city temperatures in Manchester and Toulouse

Traffic-generated heat is measurably lifting urban temperatures in cities including Manchester, England, and Toulouse, France, a new study reports.

The researchers estimate that, in simulations, this additional heat raises near-surface air temperatures by roughly 0.16°C (0.29°F) in summer and 0.35°C (0.63°F) in winter.

That extra warmth can prolong hazardous feels-like conditions when a city is already experiencing heat stress.

Streets store heat from traffic

On Manchester and Toulouse roads, the surplus warmth was attributed to routine traffic rather than the pavement surface alone.

Dr Zhonghua Zheng at the University of Manchester demonstrated how heat from engines, exhaust and braking is transferred into the surrounding urban environment.

In Manchester, the added heat was not confined to the carriageway; as conditions shifted, it dispersed into adjacent air and nearby surfaces.

On paper, the temperature rises appear small, which is why the study probes how even limited traffic heat can still become significant.

Heat beyond exhaust

For fuel-burning vehicles, a large share of energy is lost as heat instead of being converted into motion.

That wasted energy is released close to the ground by engines, exhaust systems, tyres and brakes, where the built environment can hold it near people.

“However, the direct heat produced by vehicles – from engines, exhausts and braking – has received far less attention in large-scale climate models,” said Dr Zheng.

By itemising each heat source, the method avoids burying vehicle heat within a city’s broader energy output.

Inside the climate model

To examine the impact, the team incorporated a traffic module into the Community Earth System Model, a widely used climate simulation framework.

Because the model represents exchanges between land, air, water and ice, it can situate traffic heat within a broader climate context.

“Our model will allow scientists to simulate how heat released by vehicles interacts with streets, buildings and the surrounding atmosphere,” said Zheng.

This kind of simulation enables planners to test urban design options before schemes are built, financed or required.

When heat turns risky

During Britain’s July 2022 heatwave, the simulations indicated higher values in indices used to assess risks to people.

Sustained high temperatures are felt as heat stress-the strain heat places on the body-especially when nights remain warm.

In that event, Britain exceeded 40.3°C (104.5°F) for the first confirmed time, leaving little margin for even modest additional warming.

Once hospitals, transport networks and homes are already under pressure, even a small fraction of a degree can make a difference.

Warmth reaches indoors

The heat generated at street level did not stop at the kerb, as some of it passed through building fabric and into indoor spaces.

In the simulations, indoor air temperatures rose by about 0.05°C (0.09°F) in Manchester, while Toulouse saw an increase of around 0.27°C (0.49°F).

When street heat gets indoors, fans and air conditioning must work harder to remove it.

Cities grappling with heat islands-urban areas that run warmer than surrounding land-could face greater cooling demand as traffic increases.

City design and lingering heat

In Toulouse, denser urban blocks held on to more traffic heat than the more open street settings examined in Manchester.

Narrower street canyons, greater building cover and less ground available for evaporating water slowed the dissipation of warmth.

Even with similar traffic volumes, the temperature increases were not identical, despite both cities having broadly temperate climates.

Street layout, tree cover and road materials can all influence how long vehicle heat remains after traffic has passed.

Different vehicles have varied effects

Heat output is not the same for every vehicle type, and the model can distinguish between petrol, diesel, hybrid and electric traffic.

Because electric vehicles convert more stored energy into movement, they release far less waste heat at street level than petrol cars.

Manchester’s 2022 traffic mix was dominated by petrol and diesel, with electric vehicles accounting for only about one per cent.

Cleaner fleets could slightly reduce urban temperatures while also lowering emissions, although total traffic volumes would still be important.

Planning for cleaner transport

Transport policy now has a clearer connection to local temperature, alongside established concerns such as pollution, noise and journey time.

Using this tool, planners can explore whether bus lanes, traffic-calming measures or cleaner vehicle fleets reduce heat on the hottest days.

Cleaner fleets and calmer traffic are relevant to climate adaptation because street-level heat can weaken cooling strategies.

The model also allows city leaders to weigh climate resilience alongside cleaner transport, rather than treating them as separate agendas.

Making changes to cool city streets

Even so, the analysis covered only two cities, both in temperate settings rather than tropical or desert climates.

Traffic counts, vehicle mix and street form differ widely, so later iterations will require more locations and broader datasets.

Global input data remain limited, constraining how quickly the method can expand from case studies to regional assessments.

Even so, traffic heat is no longer absent from the model, reshaping what urban climate simulations should account for.

Everyday traffic does more than move people around: it adds heat that roads, buildings and the air then redistribute.

As heatwaves become more punishing for cities, improved climate modelling could help decision-makers identify which transport changes genuinely cool the streets.