No, it is a good question really. I asked the same questions myself. I have a rosary each hanging in all my cars by the way.
I do not use them.
Yes, definitely it is for display, otherwise why hang it there.
I even asked whether it is a spiritual protection for accident.
Then I got it. For me, it is always wanting to be reminded of Jesus on the cross, who died for our sin. Then everything made sense.
The crucifx in front of me is like Jesus watching me from the cross and Mary praying for me, every time while at the wheel when I am tempted to go against God and sinning.
In a more subtle way, obviously it is a sign of Catholicism. That helps too because it is always a responsibility to bear that sign that I have to live by it especially if I should want to behave unchristian-like while driving.