I only experience this when I feel like someone who is driving the car isn't a safe driver . I feel nervous because they may be speeding or running in the front of other cars when it looks like the car is about to run into the one I'm in.
I was in a car accident about 9 years ago and although none of us were severly hurt , it was just minor things that happened , it was bad enough to scare me ! Awhile after the car accident I started to be nervous about getting into cars thinking I was going to get in an accident . But, after awhile I got over it . I sometimes still feel nervous about it . Maybe you feel this away because of an experience you had with cars and it was scary, or when you get in the car your anxiety messes with you because you are in a closed area. Either way just learn to relax and keep telling yourself cars are transportation that helps me get from one place to the other . I hope I helped , Good luck I hope it gets better for you soon!