The sign is probably an OSHA requirement. Still, I wouldn't make a "career" out of being in front of those antennas but I doubt it will cause you any harm.
Just by your questions I can tell you are spooked by this. I seriously doubt that your headaches have anything to do with the RF but more from the stress of worrying about it.
This isn't microwave and it isn't x-ray. Keep in mind also that it is not permitted to object to the location of cell sites on the basis of health concerns since no danger has been proven.
Like I said above, I would be more concerned with long term use of a cell phone itself if I were going to worry about health risks since such use results in a higher exposure than what you are talking about.
-Hal
[This message has been edited by hbiss (edited 03-09-2004).]