I'm sure that Carl will have some specific insight since he's a California officer, but I've seen quite a few people here in Florida who were quite irritated when they were arrested for doing this (and a few arrests via notice to appear tickets for people who felt obligated to flash their headlights in warning as well, although technically they were cited under different violation statutes).
I suspect any problem a court will have with your arguement that you're just trying to get people to slow down is going to come down to the number of times or amount of time you spend out with the sign when police AREN'T running radar. If you're only there when they are, it's easy to infer intent. (Particularly since it doesn't just say "slow down", but you felt the need to put "cop ahead" on it.)

