Like Tom Mallard I don't ...

Like Tom Mallard I don't know of any studies directly relating to this, but my assumption would also be that the road surface will erode / degrade over time as it is will be an abrasive running surface.

It's worth remembering of course, that roads are already a major contributor to diffuse pollution, so it will not necessarily be that they would become more polluting, but rather that the nature of the pollution will be different. Toxic, non-soluble, pervasive sediments are already a major concern in relation to highways outfalls. In 2014, the Highways Agency (as it was known then) and the Environment Agency in England estimated that 8 million tonnes of contaminated sediments from roads were entering the water environment every year.

The good thing (if there can be one) is that the existing contaminated sediments are relatively easy to remove from the run-off through the use of appropriately designed and implemented settlement systems (e.g. Downstream Defender vortex separators) or filter systems (e.g. Up-Flo Filter). Many of these have already been fitted and continue to be fitted to new and existing highways outfalls in the UK.

One of the challenges with potential future plastic micro-particles could be that their specific gravity may be very close to 1.0 (i.e. they may be fairly neutrally buoyant in water). This would require wider implementation of more filter type systems that are more costly to supply, fit and maintain than gravity settlement type systems.