Actually I think the way they got the data was fine, so long as they account for enough variables (like seasonal differences), and have enough data points. The police department information would be skewed because they are not interested in counting every car, just ones breaking the law.
Although something more interesting would be to grab the number of active cells with GPS transmitters indicating speeds of over 20 mph or so. Would give much more targeted data, although probably isn't the best precedent.
@Jae: I hadn't thought of those points. I agree with you then.