Why are many French roads turning white during the heatwave

Drivers across France have noticed streaky white lines appearing on roads across the country in the past week. 

The lines have appeared from Lyon to Brittany and in several other areas facing the hottest temperatures, leaving drivers perplexed by their sudden appearance and wondering if they are related to the recent extreme heat.

These lines are indeed linked to the current record-breaking heatwave, but are not directly caused by it.

Instead, a white mixture is being deposited by local authorities to cool roads down and help prevent traffic accidents at a time when hospitals are seeing an intake of patients suffering from heat-related illness.

What is the white mixture and how does it work?

The mixture is lime milk (sometimes called milk of lime or lime slurry), made from diluting calcium hydroxide into water. It is largely water-based (up to 85%) and contains only a small amount of calcium hydroxide, or lime.

The mixture is sprayed onto roads by trucks but the benefit is not just from the water. The white coating reflects sunlight, meaning the asphalt absorbs less heat. Combined with the evaporation of the water, this can lower the road surface temperature and help cool road surfaces by around 10C. 

It comes as roads softened under extreme heat last week. Reuters reported that in areas including Saint-Cosme-en-Vairais, Sarthe, in north-west France, a temperature of 48C was recorded on a road in direct sunlight on June 23, leaving dark, sticky tyre-like marks and deformation on the roadway.

Departments are mainly targeting roads that are particularly exposed to the sun where softened asphalt can rut, deform or shed loose chippings during extreme heat leading to potential accidents.

Although the white streaks have caught many drivers by surprise, spraying roads with lime milk is not new. Several departments have used the technique during previous heatwaves, but it is being deployed more widely this year as temperatures reach record levels.

Simply spraying the lime milk mixture and allowing it to rest over the road is enough – it requires no further treatment.

In turn, this reduces the risk of road surfaces becoming so hot that they start to disintegrate, loosening asphalt and gravel. As roads get hot and particles get dislodged they can also become slippery for cars and motorbikes. 

The lime milk acts as a cooling agent, keeping the road intact and preventing gravel or other debris from dislodging, making the road safer. 

The mixture eventually evaporates, requiring a truck to revisit to apply a new layer on the route. 

Is it safe for vehicles, and does it impact the environment? 

One of the main reasons local authorities are choosing lime milk is because of its ecological credentials, although they have been criticised for not planting more trees and hedges along roads to provide natural shade.

Studies show that the lime milk mixture poses little to no risk to human health nor biodiversity in the areas where it is used, either when it is laid or evaporates. 

“It is a chemically inert product, formerly widely used as white paint. It poses no toxicity risk and is perfectly suited for this road application,” a Rhône department spokesperson told journalists. 

In addition, the solution does not pose any risk to the vehicles driving on it.

The treatment is one of several measures being introduced during France's exceptional June heatwave, alongside reduced speed limits in some areas, train restrictions and the closure of schools and cancellation of events.