Being a Christian and sticking a fish on the back of your car doesn't make you a good/bad driver. It makes you a normal driver with a Christian faith and just as much chance of making mistakes, cutting people up driving too fast/slow, being too old/young etc. Just the same as being a Christian doesn't make you a good person, it just makes you a normal person who at least knows they are not so good and are trying to be better.
Having said all that, when driving to work in the morning and cutting people up getting in the wrong lane and overtaking as I accelerate from traffic lights, I look in my rear view mirror and see my fish and think I really should have driven a bit more in keeping with how Jesus might drive a car.