From the ACLU:
In July 2012, ACLU affiliates in 38 states and Washington sent public records act requests to almost 600 local and state police departments, as well as other state and federal agencies, to obtain information on how these agencies use license plate readers. In response, we received 26,000 pages of documents detailing the use of the technology around the country.
Read more on ACLU and read the report, You are Being Tracked: How License Plate Readers Are Being Used To Record Americans’ Movements