Where verification photos are recorded on a time sequence, allowing the calculation of actual speed, these have been used to challenge the accuracy of speed cameras in court. Motorists in Prince George's County, Maryland, have successfully challenged tickets from Optotraffic speed cameras where they were incorrectly ticketed at over 15 mph over the limit. However, Prince George County no longer allows time-distance calculations as a defense in cases where "the equipment was calibrated and validated, or is self calibrating". The National Highway Traffic Safety Administration standards for "across the road radar" state that "If the ATR device is to be considered for unattended operation, the manufacturer shall provide a secondary method for verifying that the evidential recorded image properly identifies the target vehicle and reflects this vehicle’s true speed, as described in §5.18.2. This may be accomplished by means of a second, appropriately delayed image showing the target vehicle crossing a specified reference line."