Agree with hborre the problem could be too much light is being generated by the diffuse materials.  I did an experiment with Andy in a "pit" to emulate the alley and as the diffuse lighting was decreased the contrast of the image improved (not suprising). The other point to consider is are you correctly blocking out the light that in reality would be blocked out by walls and buildings behind the camera, this also needs to be done to stop horizontal light coming into the scene (unless of course your scene would really have this light).

Was also looking to see if there was a real world guide to help in setting the diffuse values