The accuracy of cop radars is something that i have intimate knowledge
in.  If the radar is directly in front of you, then it will be pretty
accurate assuming it was tuned/calibrated correctly.  But the farther off
the side of the road the radar is, the more incorrect it will be (will say
you are moving faster).  This is because of relativistic effects- really-
even if your bus travels about as fast as a melting glacier...  Another
factor involved will be the medium that the radar 'wave' is traveling
in.  If it was calibrated in a humid place and used in a hot dry place
then it will read your speed high and the reverse.

Cheers
Matthew

Matthew Pollard             http://www.uidaho.edu/~poll7356
Dept. of Chemistry               http://www.chem.uidaho.edu
University of Idaho                   http://www.uidaho.edu