In answer to the OP (I don’t know what the other guys are talking about, and I don’t wanna know!) it’s probably true that ABS screws up the measurements.
Skid marks are useful because the coefficient of friction for rubber sliding on pavement is, according to my old college professor, remarkably constant. If you know the weight of the car, or can estimate it pretty accurately, you can figure out how much force was being applied to stop the car. From that you can figure out the minimum speed the car had to be travelling to leave a skid mark that long.
So if someone swears they were only going 30 and they left a 40 mph skid mark, they are busted!
Since ABS is supposed to keep the wheels from skidding the skid marks (if they show up at all) are bound to be shorter than they would if someone just locked the wheels. Since the skid marks only indicate the minimum speed, a 40 mph ABS skid may now only be as long as a 30 mph non-ABS skid. Or, if the ABS worked perfectly, there would be no skid mark at all. So the evidence of driving too fast for conditions would be weaker or even non-existent.
Bottom line – bad news if you’re the “Law and Order” guy, good news if you’re the perp.
“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham