Slow Down for the Speed Trap -- Slow Way Down

I was driving on I-95 in a 65 m.p.h. zone. The traffic wasn't going much faster than 65 (maybe averaging 70). All of a sudden the brake lights came on and traffic slowed down to 50. Why? There was a police car on the side of the road, probably a speed trap. I can understand why everyone would want to slow down to 65, but I still can't figure out why we had to go all the way to 50.