time = a + b*log2(distance/target_size + 1)
This is far and away the best article on Fitts’ Law I’ve seen.
The Accot-Zhai “Steering Law”, which predicts how quickly a person can “steer” something down a 2-dimensional road without “crashing” can be derived from Fitt’s Law. As you would expect, the wider the road, the faster it can be traveled. This might be my favorite implication of Fitt’s Law, since it implies that my small car may have a higher “usable speed” then a bulkier muscle car. :-p