Ling, it's partly because there are tons of races that happen on the freeway just in that area. That's where I used to go to top speed test all my cars, since it's usually wide open with minimal traffic. 2 years ago they put a police airplane in the air to track the speeders, but it's not helping much. So the cops ASS-U-ME that anyone in that immediate area with a modified "sporty" car is thinking of running it.
Also, there used to be a weekly meet at two locations just 2 miles away from that area as well, which turned into street racing activities. Every Thursday night the cops would go in full force on the streets and had 2 choppers circling the skies as well.
Regardless of all that, I still only have two words for everyone... NO PLATES!!