I had thought that ALPR data would be quite a very tool for detectives, but a related article [1] is quite disparaging about their use in Vermont. Apparently in 2013, for a ~$1m investment, 106 queries were made resulting in just 3 cases of useful information.

I think combining each states' databases as described in the WSJ will actually improve this number and is actually a prudent decision.

1: http://digital.vpr.net/post/license-plate-scanners-raise-pri...