Funny, I got pulled over & ticketed for this back in 1985 when I was 16, driving my 68 Camaro. The ticket actually said something like "flashing lights to warn of radar location". My Dad & I went right down to the police station and asked the Chief to show us the Statute that made what I did illegal. Obviously it didn't exist, so I handed the Chief the ticket and he disposed of it right in front of us.
Now, remember that there are usually laws that require you to dim your high beams within xxxx feet of oncoming vehicles. So, they could ticket you for that if you aren't careful. On my DD I have my fogs on a dedicated circuit - so I use those to flash to warn.