Answer:
There can be gravel or dust kicked back onto your windshield.
Not really, but maybe to a certain extent. A dirt road is made primarily out of dirt. A gravel road is made primarily out of gravel. But local government road maintenance departments might have a little gravel mixed in with their dirt, and a little dirt mixed in with their gravel. Also, dust and dirt can get blown by the wind onto a gravel road. Nobody is going to go out there with a vacuum cleaner and clean the dirt out of the gravel road.
