You are correct in your initial reaction, as no law enforcement officer is there at the time of the violation. The government will attempt to remedy this by taking a picture of the violation and using the picture and other evidence of your speed to attempt to convict you at trial. Often times this ticket can be fought by an attorney who can object to the foundational issues in relying on this and any other government evidence. Your attorney will explore the legal defense available to you. Contact an Arizona traffic ticket attorney so that you force the government to prove their case and not just cash your check in the mail.