Thunder reaches the car park while you are still outside. A suitable vehicle can provide shelter, but “being near a car” and “being inside a suitable car” are very different situations.
The US National Weather Service’s guidance on lightning and cars explains that the outer metal shell of a hard-topped metal vehicle protects occupants when the windows are closed. Rubber tyres are not the reason. Leaning against the outside of a car does not put you inside that protection.
Check the enclosure, not just the roof shape
Do not treat an open golf cart, motorcycle or soft-top convertible as equivalent shelter. Get into an appropriate substantial building or fully enclosed metal-topped vehicle promptly. Inside the vehicle, close the windows and avoid contact with metal surfaces and wired equipment as recommended by local lightning guidance.
The vehicle itself can still be damaged by a strike. Electrical systems, glass and tyres may be affected. Shelter for the occupants does not mean the car will necessarily be ready to drive afterwards.
Keep the other storm hazards in the decision
If driving becomes unsafe, stop at a safe location according to road conditions and local advice. Avoid floodwater, exposed trees and places where you obstruct traffic. A lightning-shelter decision does not make a flooded roadway passable or remove the risk of falling branches.
Before an outdoor outing, identify a real shelter and allow time to reach it. An open-sided picnic shelter is not the same as a substantial enclosed building. Keep weather alerts available, and follow official advice about when it is safe to resume the activity.
The useful family instruction is specific: name the building or vehicle to reach and who checks that everyone is inside. That is easier to act on than a vague promise to “be careful if the weather turns.”