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License plate reader (LPR) police patrols in crime hot spots: an experimental evaluation in two adjacent jurisdictions

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This randomized controlled experiment tests whether license plate readers (LPR) deter crime generally, and automobile crime more specifically in crime hot spots. The limited intervention tested here reflects one current likely use of LPR at the time of this publication.


We use a place-based block randomized experiment. Our subjects were 30 hot spots in two jurisdictions, 15 which were assigned to experimental conditions. The treatment involved targeted police patrols using a "sweep and sit" approach with license plate readers in these hot spots, also applying the Koper Curve timing principle. We examine effects of the intervention during and in a 30-day period post-intervention, controlling for pre-intervention levels of crime, seasonal factors, and jurisdiction.


Our findings indicate that, when small numbers of LPR patrols are used in crime hot spots in the way we have tested them here, they do not seem to generate either a general or offense-specific deterrent effect.


While we did not find significant findings of this intervention, a number of limitations and caveats to this study must be considered in conjunction with these findings. The authors suggest how already acquired LPRs might be used in ways that might increase their effectiveness in crime hot spots.

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  1. In some agencies, officers still carry paper lists of stolen vehicles against which to check suspicious vehicles prior to calling the dispatch center.

  2. ArcGIS is a product of the ESRI Corporation (see

  3. The final geocoding match rate of crime data addresses to x–y coordinates was 91.6% for FCPD and 99.5% for APD. The lower match rate for FCPD could reflect a number of factors, although we suspect it is due to FCPD’s relative newness to crime analysis, mapping, and a new records management system. It may also be due to the varied and expanded geographic terrain of Fairfax County compared with Alexandria City.

  4. CrimeStat is a free spatial analysis program available through the National Institute of Justice and the Inter-university Consortium for Political and Social Research (ICPSR). See for details on the program.

  5. There were days during the experimental period in which officers were not available, which extended both experiments in each jurisdiction further than anticipated.

  6. The start date of the experiment was delayed due to the historic 2010 Washington D.C. area snowstorm. Although most of the snow and ice had been cleared from the roads before the evaluation started, road salt and debris did affect the effectiveness of the plate readers, and snow banks blocked officer access to some parts of hot spots during the first few days of the evaluation. Another factor in the delay was the transition to a new records management system in one of agencies.

  7. The two assigned officers from APD worked opposite patrol shifts (day/evening); thus, although they may have conducted patrols on the same day, there was no overlap of coverage.

  8. One of the two LPR officers in Alexandria may not have turned off the LPR device in-between hot spots and reported plate read numbers that were unusually high on some days. Although we had the start and end number for reads for the day, we could not be sure that the LPR was not used outside the hot spots (i.e., plates read in between hot spots). Thus, the average for the number of plates scanned in Alexandria was calculated using only one officer’s reported numbers.

  9. Weekly trends of all crimes for Alexandria from the week of November 15, 2009 (“Week 1”) through the week of June 30, 2010 (“Week 32”) and for Fairfax County from the week of December 26, 2009 (“Week 1”) through the week of May 20, 2010 (“Week 21”).

  10. The large percent differences are due to the low base rates of crime in each experimental and control hot spots.

  11. Again, when including the interaction effect between the intervention and the location of the hot spot, no substantive changes were discovered, with the exception of the significance of the independent jurisdiction effect, which became less significant and greater than.05. It appears that hot spots in Alexandria either during the treatment period or after, had less auto-related thefts recorded in the hot spots, independent of whether the hot spot received or had more or less auto-related crimes during the pre-intervention period or during the year prior.

  12. International Association of Chiefs of Police (2007). Support for License Plate Reader Systems. Accessed May 24, 2010 from the International Association of Chiefs of Police website:

  13. In a recent report by USA Today, a spokesperson for ELSAG, one of the major manufacturers of LPR systems, estimated that approximately 40 agencies in the DC metropolitan are using LPR systems.


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Correspondence to Cynthia Lum.

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Lum, C., Hibdon, J., Cave, B. et al. License plate reader (LPR) police patrols in crime hot spots: an experimental evaluation in two adjacent jurisdictions. J Exp Criminol 7, 321–345 (2011).

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