Either the radar guns or the laser-based speed cameras used in Lafayette, Louisiana are inaccurate according to police tests performed in last month. Lafayette Police Sergeant Richard Rees told the Daily Advertiser newspaper that the variance in readings in a December 7 test reached as high as 8 MPH, with the speed cameras in nearly every case showing the higher reading. The city performed a second test on December 26 in light of the discrepancies.

City officials attribute the speed differential to a physical effect known as cosine error. This refers to what happens when a radar gun is not used directly in a vehicle's path. The speed estimate will decrease as the angle between the radar unit and the road increases. The fixed speed cameras use lidar units placed with a more direct line of sight to the vehicle.

Despite the cosine effect, the speed camera returned essentially identical speeds at the intersection of Johnston and South College Road, but not at two other intersections. The city could not explain how this could happen and denied there was ever an 8 MPH difference in speed recordings.

"I never agreed that all of the Redflex speeds were accurate and in talking to the officer that operated the radar he consents to only Johnston and South College." Rees wrote in an internal email obtained by the Advertiser.