Well... this is going to sound bad but I continue to drive when Cole cries, unless he is *really* freaking out. If he's losing it then I pull over but only if he's *really* losing his sh*#. Otherwise, it's "too bad, Cole - it's not going to kill you!" It's unconditional - if the car is moving, he's in his car seat, and we drive until we reach our destination, that's it. Unless, like I said, he's really crying hard - then we pull over and take him out and comfort him. We try to avoid this, though. He has gone through phases where he cries in his car seat but he has gotten over them. It is hard and you do have to limit the car trips but they do get over them. This is one of the very few instances in which I will just let him cry. I do want to note, however, that I rarely take a car trip that is longer than 15 min. - if I did, and he cried, I would probably take him out for a few minutes halfway through.
I just read this post over again and I realize I am sending a mixed message - i.e. we just let him cry, that's it! and, sometimes we stop the car and take him out. LOL. Well, that's our parenting in a nutshell. We have hard-and-fast rules that are broken under certain circumstances.