adocs,
A car is unique for the following reason:
Normally, in hilchos Yichud, one does not need to be choshesh that the situation will change (except for where Chazal were gozer to be choshesh). For example, if a man, his teenage daughter and a housekeeper are in a house, there is no Yichud because the daughter is matzil. We don’t say that we should be choshesh that maybe the daughter will go out with some friends. If that happens, then there will be Yichud, but until then there is no reason to worry.
In a car, however, the situation inside the car can remain the same but the change of circumstances outside the car can render the situation Yichud. For example, a car with a dating couple driving in Woodridge would be fine because there are other cars and people about but that same car can then turn on to a small side road and it would be Yichud without anything changing inside the car. And since it is the norm for cars to move about from place to place, maybe we ought to be choshesh.
Another issue is that the issur Yichud is rare in that it a d’oraisa that operates as fence for bigger isurrim. But what isur is Yichud meant to prevent, lo sikrevu or just the isur kareis? Driving in car makes an isur kareis unlikely but the likelihood of lo sikrevu is not reduced.
I am not saying that you are wrong or that I disagree. I am just explaining why driving in a car is a harder shailoh than typical old-fashioned Yichud questions.