Utah law is now silent about how long such data may be kept, and how it may be used.
The bill also would ban private parties such as a divorce lawyer trying to prove infidelity from using open-records laws to obtain such government records.
Sen. Todd Weiler, R-Woods Cross, the bill's sponsor, said use of such machines now help police recover 1,300 stolen vehicles a year in Utah. But he worries that as such machines are used in more places, it could allow tracking travel by people over time and become an invasion of privacy.
"It's creepy," he said. Weiler added that he went on a ride-along to watch such a machine, and found it could quickly read and store about 500 license plates in five minutes in a store parking lot.
Not only that, he said the reader could be in a car going 80 miles an hour in one direction, and read the plate of a car going 80 mph in an opposite direction at night, in the fog and in a storm "because of infrared technology."
Police want to keep data for longer periods, saying it could help solve cold cases by showing if a suspect was near a crime scene. Ogden City Attorney Gary Williams said that is "not any more creepy than what is going on every single day" by being able to track people through cell phone calls, security cameras and credit-card transactions.
But Marina Lowe, an attorney for the ACLU civil rights group, said keeping data for 12 hours would be sufficient to enforce parking violations or find stolen cars. "A core principle in our society is that the government does not invade people's privacy and collect information about citizen's innocent activities just in case they happen to do something wrong."
Two conservative groups the Utah Eagle Forum and the 9/12 Project also spoke in favor of keeping data for only short periods.
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 about how long the data may be kept.