Are road markings a source of plastic pollution?

In a training session recently, we were talking about plastic pollution, and I saw something on a graph that I hadn’t noticed before. Here’s a similar graph from the Seas at Risk campaign, showing the main sources of microplastics.

It’s the yellow curve above that jumped out to me: road markings. It hadn’t occurred to me that they would be a source of plastics at all, let alone one that significant. 7% seems really high. So what’s going on there?

First of all, yes, road markings are often plastic. Sometimes they’re just paint. Sometimes a layer of reflective glass beads is added on top. The trickier ones are thermoplastic markings, which are brighter and more durable are increasingly popular in the UK. They’re laid down by heating a mixture of plastic resins and pigments and applying it as a hot liquid. It sets very hard, and it lasts ten times as long as other forms of road marking, which saves money and disruption on maintenance. But it also creates microplastic pollution as it wears away under road traffic.

Does it create as much as 7% of microplastic pollution though? That’s less likely. The figure comes from studies that have looked at where microplastics are used in industry, and assumed that all microplastics used on the roads will eventually become waste.

However, those who work with road markings point out that wear and tear on them is never absolute. You re-apply markings as they fade, not when they’re gone entirely. They are renewed rather than worn out and replaced. The thermoplastics layer up over the years, and it doesn’t follow that all microplastics going into road markings are ending up as waste.

This was news to me in my reading up on this, but there is in fact a formal process for disposing of road markings as well. The layers on road markings build up over time, sometimes getting as deep as 10 layers. There’s a maximum to how high they can be above the road surface, which is usually around 6mm. Once you’ve exceeded that, they are removed and relaid from scratch. Removal is done with machines that blast the markings with high pressure jets and hoover up the residues, so there’s no localised waste.

Backing up this claim is the fact that there are plenty of studies into the composition of road dust, and markings aren’t an obvious problem. Neither have they been reported in studies looking at microplastics in rivers. Tyre abrasion and brake pad dust can both be readily identified, but not pollution from road markings.

A review of the scientific literature on the subject found little evidence of actual plastic pollution from road markings, with two exceptions. One was in Scandinavian countries, where drivers switch to winter tyres that handle ice and snow better. Studded winter tyres, which are a legal requirement in Sweden, take chunks out of road markings and these have been found in snowdrifts beside roads.

The other exception is where road markings are badly maintained. Instead of being relaid to keep them bright and visible, they become scuffed and faded. In those case it is likely that there will have been some microplastic pollution.

In short, road markings can create microplastic pollution in certain conditions, but nowhere near the 7% figure that is sometimes cited. One study in the literature review suggested it’s nearer 0.7%, though it’s not an easy thing to study and we don’t know for certain.

What we do know is that synthetic fibres are the biggest source of microplastic pollution. That makes the fashion industry a more important target for action than the local council road-marking team.