I used to smoke when I was younger - more than willing to admit in a car it's dangerous.
Your distracted, finding your pack, dropping one and reaching around under the seat, getting your lighter out of your pocket, using the lighter with accuracy = eyes off the road.
Once I had an unlit smoke in my mouth for too long it stuck to my lip without me realising, I lit it, went to pull it out of my mouth it stayed and I knocked the lit tip off the end which stuck to my finger and burned me badly and was well distracted - still have a big scar it burned me pretty deep.
While I realise this makes me sound like the clumsiest person around, every other smoker I know has a few of these stories and that's all it takes.
As for passive smoking - once you give it up you realise just how filthy it is and how uncomfortable it makes every non-smoker around you.