word choice - "In" and "on": How can I decide which one to use for vehicles?

Examples:

• In a car, van, etc.


• On a bus, boat, motorcycle, etc.

How can one decide which preposition to use? Is memorization the only way or is there a better way?

Note: People generally explain this by either distinguishing between open and closed vehicles or between large and small vehicles. However, the examples I've given defeat both explanations.