Need Help to stop child from hitting me

When my son was young and we had major, major tantrums, if we were out, I would leave the store we were in and we would go sit in the car. (a few times we went home, but he seemed to like to throw them when I had driven an hour to get to a bigger store, as we live in a very small town) I would fasten him into his carseat and get into the front and start the car, and turn the radio up. Sometimes we just sat there and sometimes I drove around. (if a lot of people were staring at us) When he calmed down we tried again. But when he started again, we would just leave the cart and walk out right away, sometimes the shock would be enough to calm him down before we got very far. After a while just asking him if he needed to go to the car was enough.