Here in Ireland, we have a complicated relationship with hot weather – we’re desperate for a few sunny days until they come. Then half of us immediately pivot to complaining about the heat.
While it makes a change from the wintry or rainy weather that’s usually the focus of road safety alerts, hot weather brings its own problems on the roads – not least of which is the problem of road surfaces melting in the heat.
A few years ago in the UK, a section of the M25 – London’s equivalent to the M50 – had to be closed because the surface was unsafe. Predictably it caused chaos. Closer to home, we have had reports of melting tar in some parts of Ireland during a really hot spell in 2021, including many in counties Galway and Sligo.
It may seem crazy that our roads can let us down like this. It has been hot, of course, but there are roads in warmer countries that get this all the time without disintegrating, and motorists have been asking how on earth it could happen.
It is a bit like the reverse of our problem after the “Beast From The East” and Storm Emma in 2018. Irish roads, especially secondary roads, took damage from the icy conditions. Again, those low temperatures do not cause problems in Norway, so why here?
Why don’t roads melt in hot countries?
The answer is that roads are built within ‘normal’ design parameters that will vary from country to country. Ours are made to deal with much more rainfall but much less heat than a road in Nevada, for example.
The mercury tipped past the 30-degree mark recently, but it only ever gets that hot for a few days at a time, and not every year at that. You can build roads that can cope with those temperatures using special polymers and binding agents in the top layer of the tarmac, but it is expensive. It is not really possible or sensible to over-engineer a road to such an extent that colossal cost is incurred, making it proof against problems that arise very rarely. This is especially true for roads that are not heavily trafficked, as the level of damage and risk may not be as high.
The road surface can be as much as 20 degrees hotter than the air temperature, usually measured in the shade and above the ground. If you are curious, you can look at the road temperature map on the TII website. Their thermal mapping system was put in place with winter ice and gritting in mind, but we have seen road temperatures topping 50 degrees in the heatwave.
Does tarmac melt in hot weather?
When it gets as hot as that, the tar on the road will start to behave like molasses, “melting” or softening. As traffic uses it, the road will develop ridges and ruts like a mud track and become sticky. For the unsuspecting driver, you can hit a bad patch without warning. Braking and steering will be affected, and for those on motorbikes, it is extremely dangerous. In extreme cases, it could also coat tyres and damage your vehicle.
In some areas, roads have been treated with grit or gravel. We are all familiar with the safety advice for the driver: slow down and expect the unexpected. Reducing speed and staying clear of the car in front is the only effective countermeasure, and any problem will be magnified if you drive too fast. If you can see a road surface ahead has significant heat damage, it’s best to avoid driving over it and seek alternatives.
What temperature does tarmac melt?
Temperatures rarely reach the point where the tarmac would melt. However, it can soften and deform under very high temperatures, typically above 50 degrees. The tarmac temperature is hotter than the air temperature.
Finally, another word of warning. In the Mediterranean heatwave of 2012, a huge forest fire took hold on the French/Spanish border north of Barcelona. Thirty-two thousand acres were destroyed, four people died, and over 100 people were injured. The hugely busy AP7 motorway was essentially destroyed.
It is suspected that the disaster may have been caused by a driver flicking a cigarette from the car window. If you’re driving in Ireland currently when there’s a status orange fire alert, now be extra cautious with all sources of ignition.
This post was originally written by Chris Jones in 2018 and updated for the 2021 heatwave by Lauren Beehan.