A good friend of mine lives in King of Prussia, and used to fly out of BWI (ATA). He said that he leaves 2 hours for the drive, but usually makes it 1:50 or so. Since we are usually not driving in the normal traffic times, he said he rarely sat in traffic. The only time it would be an issue is if you are on reserve with a 2 hour call out. It would be pushing it if you sat at home.
HTH!