Having watched from the passenger seat as the driver dropped a lit cigarette into their lap and then fight to find it and get rid of it by wildly thrashing about with both hands before flames took hold while driving at 30mph, I think that there is good reason not to smoke.
There's already a massive number of distractions in most cars, I don't think eating a chocolate bar is an issue. However, they're not the easiest of things to open requiring both hands.
As indicated, bananas can be opened using one hand and teeth. So, does doing opening a banana or mars bar pose a danger. For some I suspect it will - The thought of somebodies top-set flying out and getting stuck under the brake pedal would be difficult to explain in court.