Speed limit in the US

Today I read an interview with John Cleese, who now lives in the US. One of the things he told, is that one day he was driving on a crowded road, carefully keeping the speed of his car somewhat under the speedlimit that was indicated clearly.

But then a policecar stopped him, because he was driving too slow, slowing down the traffic. He was very surprised, and explained that he carefully staid somewhat under the speed limit. The cop replied that the signs with the speed limit are only advisory!

Funny, but somehow it makes sense. If you move along with the traffic, you're less likely to have an accident than when you're driving a lot faster or a lot slower.