Your link says that 2.5 is to allow for worst case situation and below average drivers.

If all but the slowest 10% can react in 2.5 seconds than I would think many would do a fair bit better.

Edit: Apparently the average person is closer to 1.1 seconds. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.372...