Potholes aren’t just annoying – they damage cars and can be deadly for cyclists. And they are proliferating, as cash-strapped councils spend less on road repairs.
Could the solution be new road designs that stop potholes from developing in the first place? There are several high-tech approaches in the works, from self-healing asphalt to roads with inbuilt heating systems that stop ice damage.
And yes, if anyone doubted it, potholes really are getting worse. The number of these road hazards reported to local councils in 2023 was at a five-year high, according to a survey by environmental campaign group Round Our Way in January.
The RAC has also said the number of callouts from motorists due to pothole-related breakdowns was a third higher in 2023 than in the previous year.
Perhaps it’s unsurprising considering the falling spending on roads by local authorities, which are responsible for all roads except for A roads and motorways. The amount spent on UK local road repairs halved from £4bn to £2bn between 2006 and 2019, according to an analysis last year by the Local Government Association (LGA).
“Limited resources and a £16.3bn repair backlog means councils continue to prioritise road repairs according to local circumstances,” said Councillor Adam Hug, transport spokesperson for the LGA.
Brace yourself, however, as the pothole plague is likely to intensify over the next few months. Rain and wintery weather are big contributors to the road hazards.
Potholes begin as tiny cracks in the asphalt, on the road surface, that let in water. Freezing conditions turn the water to ice, which expands and makes the cracks worse, especially once ice begins to form underneath the road, creating holes under the asphalt after it melts.
A repeated cycle of freezing and thawing, together with heavy wear from traffic over the crumbling asphalt, means that once they start, potholes tend to get bigger and bigger.
One approach is to try to stop the hole from developing in the first place. Transport Secretary Louise Haigh recently urged local authorities to copy Blackpool Council’s pothole prevention scheme.
Blackpool’s strategy includes driving vans around the road network and using cameras to scan the asphalt surface, so emerging potholes can be repaired before they have a chance to grow – even when they are just cracks. “If you intervene early, you can prolong the road’s lifespan,” said Ian Large, the council’s head of highways and traffic management.
Self-healing roads
But there are more innovative approaches in the works. Several teams of engineers have been developing additives for asphalt that could make it more resistant to cracks.
Asphalt is made of gravel or crushed stone mixed in with bitumen, an oily substance that liquefies at higher temperatures, which is why roads can become slightly soft on hot summer days.
If the air temperature gets above 40C, any tiny cracks in asphalt tend to naturally seal up themselves, according to Dr Jose Norambuena-Contreras, an engineer at Swansea University. “The bitumen can flow,” he said.
Temperatures almost never get that high in the UK, so Dr Norambuena-Contreras and others have investigated adding micro-capsules of oil to bitumen during the asphalt manufacturing. The idea is that any cracks would break these capsules, releasing the oil, which would make the cracks seal up.
Cracks should be more likely to form where the capsules are, because that is where the material is the weakest, said transport engineer Professor Nick Thom at the University of Nottingham. “If a crack appears, it’ll go for where it’s weak.”
The first tests of a road surface embedded with such capsules was promising, showing less stone loss after two years, suggesting it is more hard-wearing. Now we need to wait for results from a real-life trial, where the new material has been used on a section of the M26 in Kent, along with a control section.
“If there are real differences they should appear five to 10 years after construction,” said Prof Thom. The trial began in 2022.
Warmer roads
A more high-tech approach involves trying to stop the freeze-thaw cycle that drives pothole formation, by using pipes of water to transfer heat from the warmer soil beneath the road up to the asphalt at the surface.
This would require two horizontal layers of pipes, one within the asphalt and one in the soil, up to 5 metres lower, with heat exchange pumps to connect them. The system would also work in reverse in summer to cool the asphalt by taking its heat to lower levels, which would stop asphalt softening and reduce the street’s air temperature, said Dr Benyi Cao, an engineer at the University of Surrey.
This approach is at an earlier research stage and it may be relatively expensive. On the other hand it could be cheaper in the long term as the costs of maintaining and repairing our current roads is £2m per mile over a typical lifetime of 20 years, said Dr Cao.
Porous roads
Road builders in the Netherlands are able to block the freeze-thaw cycle a different way. They have started making some roads with materials that are fully permeable, so rainwater neither sits on top, nor pools underneath the asphalt.
But this is being done mainly to combat flooding, rather than cut repair costs, said Professor Floris Boogaard, a climate adaptation researcher at Deltares, a research institute in Delft. The water has to drain into pipes underneath the road and over time these need regular maintenance to avoid getting clogged with silt.
It can be easier to repair any potholes that do arise, though, because the Dutch permeable roads are usually laid as bricks, and so these can be easily replaced, he said.
Prof Boogaard believes the UK’s pothole problem is chiefly caused, not by our different road designs, but lack of repairs. “Maintenance is key,” he said. “It’s mostly about your budget.”
What causes potholes?
Road surfaces are always going to give way eventually, due to the heavy wear they get from traffic. But another big contributor to potholes is winter weather, namely rain and freezing conditions.
Potholes begin as tiny cracks in the asphalt that makes up the road surface. This lets water get in, which can freeze when temperatures drop. Ice takes up a larger volume than water, so as the water freezes it expands within the cracks, making them bigger.
The next stage is when the cracks get so big that water can reach the soil underneath the road surface and pool there. When this water freezes and expands, it can push up the asphalt. Then, when it gets warmer, the ice melts and evaporates, leaving a hole under the surface.
When cars drive over these areas, the asphalt then collapses, causing a pothole. In a typical British winter this can happen repeatedly, sometimes with the water freezing every night and thawing every day, said Dr Benyi Cao, an engineer at the University of Surrey.
“One day can form a freeze-thaw cycle,” he said. “The heavy rain and the freeze-thaw cycle are definitely two big contributors.”
