Simple math... if a driver in a 45-MPH zone passes A at 5 pm and 1 minute later, passes point B 1 mile away, how fast is he travelling? And more importantly, is he speeding?
Well, this would be more relevant in metric, but we'll save that for another day. In UK they have cameras that track you as you travel. If you pass two points too quickly, then you're obviously speeding. A major flaw? You can outwit the system by simply changing lanes.
That seems like a major flaw.