Is there any single word that signifies both two and four wheeled vehicles. i.e. Cars and Motorcycles?
A wheeled vehicle is a general expression you may use:
- a vehicle that moves on wheels and usually has a container for transporting things or people.
The Free Dictionary
A vehicle seems a likely choice--though it technically could mean any vehicle (boat, auto, motorcycle, space-ship or plane) you're most likely to hear it used with land vehicles.
Examples: "Please step away from the vehicle." or "Please step out of the vehicle."