So, is "met de auto rijden" to actually drive the car yourself or is it just to go by car, not necessarily as the driver?
Exactly. Wouldn't you translate "You may not drive" as "Je mag niet rijden" as you might not be driving a car, it could be a lorry or motorcycle.
I suppose there is a slight problem as "Je mag niet rijden" could translate as "You may not ride [a horse]", so using "drive" shows that we have spotted that is a car.
But I don't think it is wrong in English to put all the information about what you are driving.
david_hinter, the English word "may" has different meanings:
1. may = perhaps = misschien
2. may = have permission to = can = mogen
An example of 1: "He may come, but then again he may not."
An example of 2: "You may smoke if you go outside."
The DL sentence here is about permission, not possibility.