Well ok, talking in theory;
You get hit by a car it might just run over your leg or your arm or just paralyse you for life so you can't move or talk which would be truely horrendous, real torture just being a vegetable. So hit by a car isn't a definitive death.
I would say jumping off a cliff (100m +) would be definite death when you hit the bottom. So in terms of more certain, cliff jumping.
Speaking of other ideas, well there always potassium cyanide or sodium cyanide you would need probably not even a teaspoon in some water for surefire death. You would become unconscious in about1 minute and then die within an hour. I can't remember how it causes death but I think it starves your cells of oxygen or something, anyway not really a nice way to go. Anyway, its very very difficult to acquire any of that stuff.
So all in all I would say cliff jumping would be the best option, at least (one would expect) on top of a cliff there should be a nice view in a natural environment and a bit of peace where you can meditate a bit and think through life, calm down and decide what you want to do. Especially at night time there should be a beautiful starry sky to look at if it's not cloudy.
I mean I think it's nicer than standing on a polluted freeway/motorway deciding which car to jump under and probably causing a pile up unless you're trying to kill other people.
When I say 'you' I'm talking in general, I could use 'one' but then I'd be saying 'one' all the time which is a bit incoherent.
Before anyone says I'm encouraging suicide or whatever, I'm not; I'm just answering the question.
Ok, I've just read your additional details: I suppose in reality the greatest control you will ever have over your life is by killing yourself (think about it; every other aspect depends on something). I mean people do get sick of life, I do occassionally. It's your life so do what you want with it. It's like one's obliged to live even if one isn't happy.
It would be best to have some peace before you go, so I suppose a cliff would be the most peaceful setting.