This is definitely a thing - though finding a therapist who will actually leave the office with you may be difficult. It may not even be about boundaries so much as logistics / insurance difficulties. Still, I'd definitely bring up the phobia with your T - they may be able to help without actually being in the car with you.

I hope you find something that works. I was terrified of driving on the freeway - as a matter of fact, I didn't do so until I was 26 and sent out of town for work...and, well, I just had to. It was awful, but I got through it. I hope you can get through your phobia, too.