I would suggest that the edge of a typical busy boulevard features 10-30 thousand vehicles per day, whose brake and tire particles in particular end up in the soil around the road. Typical farm fields might see 30,000 trips over the span of what, 100 or 1000 years? It's not in any way comparable to an urban roadway.
It was erosion control after some new road work. I sorta suspect they did bring in new dirt for it as well. I do only use it in small amounts.
Not really gonna make a difference over time in terms of pollution/heavy metals. But it was still a worthwhile project.