When I see a Jesus Fish on vehicle on the road, I avoid that vehicle at all costs. On the road, it's not a sign of a believer to me - it's a sign of doom.
It's a sign of driving down multiple lanes. Sometimes even three lanes simultaneously. It's a sign of changing lanes without a turn signal, while I occupy the very space being turned into. It's a sign that I'm going to be tailgated in the fast lane, and also a sign that that I may have to drive 20 mph under the speed limit in the fast lane - and those two things also may occur simultaneously.
In the parking lot, it's a sign that you can take up as many parking spaces as humanly possible. It also may be a sign that I'll never ever get to park my own car, as that symbol means you'll wait for ever in the parking lot traffic lane for that sweet spot that that family of five is about to vacate. Here, you may or may not use your turn signal - but probably not until I'm too close to you to get around you.
The Jesus Fish (or Ichthys, if you want to get technical) is an ancient symbol of Christian believers, that also makes a fantastic sticker for the car, where everybody can let everybody else know that they believe in Jesus. Showing your faith on your vehicle is not the problem here - it's showing your faith and then driving without care for others that is.
Just think about it ... Christ calls us to live like Christ all of the time ... That means maybe not driving like a Jesus Fish bumper sticker gives you a free pass to drive like a (fill in the blank). So if you have a Christian symbol on your car, maybe you should think about how you drive.
Have you ever thought about how Jesus would drive? He probably wouldn't drive at all. He'd probably take the bus. More people to talk to that way. And he could avoid the Christians driving like maniacs.
Drive now in peace.