I disagree, BeautifulMadness, you never HAVE to let them "cry it out" - but waiting until you can pull over is acceptable. Because stopping dead in the middle of traffic, jumping out the car and then you find that the car behind has run into you is a far worse solution.

I found that no matter why DS was crying, The Cat in the Hat CD stopped his upset and sent him to sleep. Usually I'd walk everywhere and if not then DH would be driving and so I could sit with DS or at least play peep-o with him, which he loves even now. I have driven with him crying, I would sing to him usually and that calms him a bit, although it's not ideal.

And a feed and nappy change before a car journey of more than 20 minutes when he was tiny worked wonders too.