He said such machines can read and store data on about 500 license plates in five minutes in parking lots which has helped police in Utah to recover about 1,300 stolen cars a year. They can also read license plates on cars passing at high speed in the dark because of infrared technology.
If the data is stored over time from many locations, he said it could allow tracking movement of motorists over time.
"It's creepy," Sen. Mark Madsen, R-Eagle Mountain, said in earlier debate.
The bill also would require a court order for access to the data from people outside of law enforcement.
Police had wanted to keep data for longer periods, saying it could help solve cold cases by showing if a suspect was near a crime scene. But the American Civil Liberties Union wanted it kept for essentially only a day to prevent any long-term tracking.
Weiler's bill comes after a firestorm last summer created by a U.S. Drug Enforcement Administration request to put readers on freeways in southwestern Utah to record the license-plate numbers of all vehicles there to help catch drug runners, as it does in drug corridors in Texas and California.
Public outcry led DEA to withdraw the request. But lawmakers learned that numerous police agencies already are using mobile license-plate readers leading to Weiler's bill.