Things can go three ways—left, right, or straight. If you stand on one side, either the left or the right, you will have the quickest reaction 1/3rd of the time, the second quickest reaction 1/3rd of the time, and the slowest reaction 1/3rd of the time. But if you stand in the middle, you’ll have the quickest reaction 1/3rd of the time, and you’ll have the second quickest reaction time 2/3rds of the time. I guess things can also go backwards, but the middle is still the best place to stand.