Leashes are pretty iffy in a car, since it can get caught up on say the shifter if he were to move for some reason. My parents use a body harness that connects to the seat belt for their border collie, but if mine isnt out the window, he is passed out on the back seat. I think it really comes down to how the driver behaves and thats reflected in the pet. I try to be as safe as i can driving, rarely speed, and if my dog is impeding me, he will get a smack to the head and get back in his seat.