I think that smoking in one's own car while alone is similar to eating or drinking in the car. You are taking the risk that you'll drop the cigarette. If you eat, you're taking the risk that you'll choke on something or distract yourself by dropping food in your lap. Or maybe your greasy fingers will slip on the wheel, who knows. The point is that we expect adults to be able to gauge risk, and we should leave it to them to do so. Heck, I bet having screaming children in a car is more distracting than eating, smoking and cell phone usage combined!
I am allergic to smoke. I watched my father die at age 67 of cancer due to smoking. I believe that smoking is a dumb thing to do given the known risks. It's still a choice people should be able to make.
If the driver of a car is smoking and has their window open, trust me, there is still plenty of smoke in the back of the car. My father smoked for years while driving and I still got headaches from the smoke as I sat in the back.
Smokers can't smell themselves. I can smell the tang of smoke on almost any smoker if they've just had one. Some people I can smell all the time, I'm assuming they either smoke more or don't change clothes as often.
If I visit a smoker's home, even if they've fully vented the place for hours prior to my arrival, I need to shower and wash the clothes I wore when I get home due to the reek of stale smoke. I never mention this to my hosts, so they probably think their place smells fresh as a daisy. It doesn't.