Tom, I have a client with a number of driveways where we are using 5MP cameras at distances of 80 ft to 120 ft and capturing license plate images. We had three situations where we couldn't get clear plate data.
Angle of Plate
If the car is turned so the plate is at a 45° angle to the camera at the farthest distance, it could be hard to distinguish some plates. But pretty much straight on, there wasn't a problem.
We did have an issue initially where the installer focused the cameras in bright daylight (long depth of field) and then in low light conditions (short depth of field) we discovered that the focal point was not where it should have been, and plates in the target zone were very fuzzy. Refocusing the cameras during low light conditions and with a plate in the target area, we were able to get the best results and more than acceptable.
When the vehicles were moving at 45 MPH past some of the fields of view, we got motion blur on some images, but could still make out the plate numbers when we looked at them at full resolution on screen.
Of course, when we had 4 viewing windows up at once on the monitor, it could be hard to distinguish plate numbers due to the image downsizing. But viewing the image at full resolution and centering on the plate, the plate numbers were big and clear.