Cutting your throat is completely lethal (if done properly). And many people have done it. I think it was quite common in the 19th century, when most men used the old fashioned kind of razors. (They were called "cut throat razors" for a reason, probably because it was all too easy to injure yourself with them even when you didn't intend to.) A famous example was Robert Fitzroy, who captained HMS Beagle when it took Charles Darwin around the world, and much later in life cut his throat.
But if you are worried about whether it is fast enough that you won't have time to regret it, then I don't think your should be catching the bus at all, by this method or any other. If you consider that there is any possibility that you might regret it, then it is not time to go.