Calculus: Why is this related rate problem ill posed?

Vt = πd/t = 135π/20 = 21.206 m/s, which is a whale of a delta-v to step across. If the wheel revolves continuously, it does not stop to embark or debark pengers, and tangential acceleration is always 0. If it stops for pengers, there is insufficient data to determine acceleration and deceleration.