Not only are they compressed, but many radio stations speed up their songs by 1.5% to make the other stations sound like they are dragging.
This is from www.omniaudio.com Omnia is one of the many companies that make processing for Radio and TV:
First read some background here:
http://www.omniaaudio.com/techinfo/retro/default.htm
Then from their product page:
"Successful programmers don't like to compromise. They need loudness to stand out from the crowd and grab button-happy listeners, but they need clarity to keep listeners from becoming fatigued and tuning out.
That's why leading stations around the world choose Omnia. Omnia makes it possible to achieve unbelievable loudness without sacrificing clarity. Nothing else can compete. Omnia enables programmers to get the sound they need to increase TSL [Time Spent Listening] which means better sound, higher ratings, happy listeners.
5 of the top 6 most-listened to stations in America use Omnia.* Not only that: 4 of the top 5 New York City stations rely on Omnia to help keep their listeners tuned in longer. So do major international broadcasters like Skyrock, the BBC, NRJ and Swedish radio. Proof positive that people who demand only the best demand Omnia."