When smoking you can still use both hands on the steering wheel and if a heavy smoker went without and got ratty and aggressive due to not being able to smoke on a long journey the aggression shown through their driving and road rage etc could be worse than if they kept their nicotine levels steady.
But an alcoholic would get tremors if they hadn't had a drink and a heroin addict gets the rattles when no drug is in their system, shall we allow them to have a drink or score a hit whilst they drive? If addicts are not safe on the roads due to withdrawal effects experienced through not using substances surely they shouldn't be driving full stop, not that they should be allowed to use drugs whilst driving because they can't go without a cigarette.
I don't think smoking and driving should be allowed at all. I beleive there was movements to ban smoking in cars with children in it which goes some of the way. But I don't think anyone should be allowed to,it just seems unsafe to me.