content: Plastics (especially PET from things like bottles) will wear at a higher rate than the crushed stone used in asphalt roads (asphalt is a combination of bitumen and gravel or sand). Most plastics are susceptible to UV damage at a much higher rate than stone, in addition to being effected to a higher degree by chemicals (think things like oil, grease, automotive fluids, de-icing solutions, etc).  Additionally, let's not forget how much the average vehicle weighs. Tons and tons of automobiles and trucks rolling over it everyday is going to cause a lot more stress to the plastic than the rocky road. And what about the forces that are created when there's an accident?