Growing up, my father would often be driving my friends and myself to different places - baseball, swimming, basketball, whatever. He would always say that he wouldn't start driving until everyone had their seatbelt on, much to the dismay of some friends, particularly those whose own parents weren't as insistent. I never understood the other parents or kids - it's not a big deal to put on a seatbelt, and it's pretty clear that it can save your life. I was happy when I was in Israel and they finally started putting out really powerful ads on the radio and TV encouraging people to wear belts, seeing as more people die in car crashes than terrorist attacks each year there. And now, I'm happy to see this excellent ad (posted on Chaim's blog) by New Jersey Governor Jon Corzine, who was almost killed in a car crash a few months ago because he wasn't wearing his own seatbelt. It's simple, honest, powerful, and to the point.
Another small note: Unbuckling for "just a few seconds to grab something from the back" in a moving car is extremely dangerous. We know people who have been hurt badly in those few seconds. Moreover, just the fact that you're reaching around makes it that much more likely that the driver will be distracted or otherwise unable to see and get in a crash. Wait until the car is stopped, and don't start driving again until the person is buckled. The three seconds of waiting won't kill you; riding without a seatbelt might.