Suppose that an observer knows that his vehicle detector turns on and off before the vehicle has completely crossed the detector. From experimental calibration of the instrument he has verified that the true length of a vehicle lj is always ? larger than the effective length for all vehicles (independent of j). He knows ?. He observes a flow q and an effective occupancy r' from his detector. What else must he know in order to convert his r' into r so that he can compare his observations with someone else's? (The vehicles have different lj and vj).