Removal of privilege is usually the next step after time out. So loss of eg TV time, pocket money, earlier bedtime etc.
Or you have some kind of points method eg pasta jar where you put pasta in for good behaviour and take it out for things you don't like, then when it's full they get a prize. The idea is more positive than negative but it will fill more slowly if they aren't behaving well.
But in general I don't think punishment is that great of a (de) motivator, can you think of a positive behaviour to encourage, or try to see his side of the issue?
When/why is he running off? Does he have good road sense in general? (Maybe not if he sits in the road!) Can you just ignore it so it isn't getting a load of attention? Why does he want to take the car, how far is it to walk, is it hurting him, do you need to get his feet measured/check for flat feet? Bag too heavy? Is he cold?
Can you make the walk fun for him. Invent a game. Get a step counter and see how many steps you can do (not if this would induce health anxiety) DS1 and I used to find a conker and see if between us we could kick it all the way home after school. Tell a story where you make up one sentence each and it continues only on the walks.