The law says
Four simple criteria define normal cars and trucks, said Sergeant Michael Maffei, a veteran of the Cambridge Police Department's traffic unit. If state law requires your wheeled contraption to be inspected, registered, and insured, and to be driven by an appropriately licensed driver - e.g., for a motorcycle, you need a motorcycle operator's license - then it's an ordinary "motor vehicle." Every word of Massachusetts General Laws Chapter 90, the law of the road, applies to you.
"If it doesn't check off those four boxes," Maffei said, "it's an exception."
Motorized wheelchairs are one such exception. The law acknowledges their existence, but prohibits them from being driven on roadways as vehicles, because most travel only about 5 miles per hour. (A person in a motorized wheelchair is considered just another pedestrian, Maffei said.)
Other "exceptions" are allowed on roadways, albeit under their own special, and often screwy, rules.
Take mopeds, for instance. Mopeds don't need license plates and don't need to be inspected, but you must have a driver's license to operate one, and you must wear a helmet.
You can drive a moped on the street, but not on a divided highway, and never faster than 25 miles per hour. You can drive a moped in an on-street bike lane, but you can't drive a moped on an off-road recreational path, such as the Minuteman Bikeway.
The difference between a moped and a motorcycle? If your moped's engine is less than 50 cubic centimeters in size, it's considered a "motorized bicycle." Even if you run a red light, the maximum fine you can get is $25. If your moped's engine is bigger than 50cc, the state considers it to be a motorcycle, and thus no different than a car