Most of the trash is empty beer bottles, fast food containers and empty energy drink bottles. Most of this is done by people who have no idea what a garbage can looks like and leave their shopping carts in an open parking space.
PASSENGER: "Dude, don't worry, when we get off the highway to stop at that all night adult bookstore, lets throw them out on the blind curve on the off ramp".
DRIVER: "Good thinking! Hey! I just popped a boner, can two people fit into a peep booth?"