MWM... I'll chime in here, as traffic enforcement is my specialty. Pace clocking, as many have already stated, is a valid way enforcing the speed limit. Using the speedometer of the officer's vehicle is a means that this is done by, by using one fixed object to the next and looking at how fast the patrol vehicle was going to a point where they were staying in pace with the violator's vehicle (neither lagging behind, nor catching up). The only time I like this method of enforcement is when it's done from a patrol car equipped with a radar. Radars have the two modes, moving and stationary. When a radar is switched into the stationary mode, a ground speed is displayed in the target window, which should equal the speedometer speed of the police vehicle, and it's easier to articulate this, both to the violator and in court, if the officer had this available to them.
Aerial enforcement is my specialty, as I'm a pilot for a police agency. To me, this is the absolute most accurate way of determining vehicle speeds, because you're using simple math. Yes, that's what the white marks are for on each berm of the roadway, at least here in Ohio. These are typically mile to mile and a half zones, with each section broken down into quarter mile sections of roadway, each section identified by white berm markings. The stop watches of the pilots are checked monthly against the Atomic Clock at the Naval Observatory in D.C., and daily against a comparison calibration chart.
There's a formula that is basically an off-shoot of the time/speed/distance calculations.
Just for conversation's sake, because I know right away that if you travel through a 1/4 mile at 11 seconds it will equal 81 MPH, I'll show how we determine the vehicle speed:
1,320 (distance of a 1/4 mile) / 11:00 (elapsed time) = 120 Feet per second
120 feet per second * 3,600 (seconds in an hour) = 432,000 feet per hour
432,000 / 5,280 (obvious, LOL) = 81.818 MPH
I love this method of checking vehicle speeds because it gives every advantage that can be given to the violator, in terms of accuracy. Even if I were to short change the violator's 1/4 mile by 13 feet, a 1% error, it would only throw the violator's speed off by less than 1 MPH. So instead of doing 81.818 MPH, they would have been doing 80.9 MPH. If they wanted to argue that they were going the posted speed limit, let's say 65 MPH, then I would had to have missed their vehicle by over 300 feet for them to have been going 65 MPH.