The fastest time for running 100 m is about 9.8 s. So, someone running that fast would have run 100/9.8 m/s, or about 10.2 m/s, which is about 36.7 km/h. After taking acceleration into account, I would guess that the world’s fastest person could run about 38-39 km/h.
The world’s fastest man Usain St. Leo Bolt has a record of running 100 meters in 9.58 seconds from the 2008