Presence detectors on highways record whether or not there are any cars on some nonzero length section of highway. The absence of cars is used as a criterion for switching a vehicle-actuated signal. The problem considered here is the following. Cars pass the detector according to a Poisson process, at velocities that are independent, identically distributed random variables. Starting at some arbitrary time origin, how long will it be before the detector of length L is empty for the first time. It is shown that, in most cases, this time is nearly the same as if all cars had the same velocity q/k, q = flow, k = spacial density, and is rather insensitive to the distribution of the velocities.