What would worry me more is when the crap becomes *un*-lodged. If someone were travelling around a curve and picked up a significant amount of gravel, it would be flung at speeds of up to twice that of the moving vehicle. A piece of gravel as big as the end of your finger would crack your skull if it hit you at 200 km/h.
Also, driving through puddles could be a *really* interesting sight. I can see the tweels acting as paddle tweels, hurling thousands of gallons of water in a very dramatic fashion.
They're going to have to invent some type of durable skirt or cover to avoid these issues.