"At what speed does he run?"

Translation: À quelle vitesse court-il ?

[Alde.bj]

À quelle rapidité court-il? Why this sentence wasn't correct.

[Sitesurf]

And "la vitesse" is a unit of measure, which "la rapidité" is not.

[Ripcurlgirl]

A difference in nuance, I suspect. Rapidité tends to be used in a figurative sense meaning "quickness", speediness", "promptness", "briskness", "swiftness" etc of service or response to a letter or request.

On the other hand vitesse is used more in relation to actual speed, velocity and physical movement.

[PredragZec]

What kind of question is this? Yeah right, I always carry a radar to mesure the speed of random people...