Rural Data Collection Technology - FHWA Operations - U.S. ...

2.4 Automatic License PlateReaders (ALPR)At a Glance• Optical cameras capture images of licenseplates and software “reads” the information• Mature technology (over 30 years)• Installed above the roadway and requiredirect line-of-sight• Particularly sensitive to factors thatreduce visibility• Potential privacy issues are a concernHow It WorksOptical cameras capture images of license plates ofoncoming or receding traffic and use video imageprocessing to “read” the license plates. License platenumbers can then be matched at sensor locationsdownstream to generate travel times. Cameraimages can be stored, though this is not requiredfor travel time data generation. While ALPR is oftenimplemented as a stand-alone function, it can alsobe considered as one potential function of a machinevision system (see Section 2.5). One advantage ofALPR is that nearly all vehicles have a license platethat can potentially be observed.State of the TechnologyALPR is a relatively mature technology that has beenused for more than 30 years, though technology hasimproved and become substantially less expensivesince the earliest implementations. ALPR has beenused for tolling, law enforcement (e.g., detection ofunregistered/stolen/warranted vehicles, automatedspeed enforcement), and real-time travel time.Hardware and InstallationALPR uses cameras that operate in the visible lightspectrum. Cameras require direct line of sight tolicense plates, so they must be installed above theroadway or on the roadside in locations that minimizevisual obstructions (e.g., from surrounding traffic) andavoid off-axis angles that could reduce recognitionaccuracy. To trigger image capture, cameras can usevideo image processing to detect vehicle presencein the frame, or a separate presence-detectiontechnology can be used (e.g., inductive loop). Atnight, cameras may need additional visible or infraredillumination for adequate license plate recognition.Figure 6 shows an ALPR camera and illuminatorused in Arizona. Current systems most often useone camera per lane for license plate recognition.Implementation ConsiderationsALPR can be used for a variety of purposesindividually or simultaneously. For instance, it cantheoretically be used to detect unregistered vehicles(i.e., cross-reference license plate readings againstvehicle registration database), conduct averagespeed enforcement (i.e., detect an individual vehicle’sexcessive speed over a road segment), serve as aclosed-circuit television (CCTV) feed, and serve as atravel time data source all at the same time. Becauseit relies on a clear view of license plates, however,ALPR is particularly sensitive to any factors thatreduce visibility, such as precipitation, lens fog,line-of-sight obstructions, low ambient light, off-axisviewing, and license plates that are dirty, obstructed,missing, or have low character contrast. In a workzone implementation on a two-lane rural highway inArizona, ALPR achieved a 60 percent recognitionrate and a segment license plate matching rate of11 percent, which was considered sufficient forthis implementation (FHWA, 2004). Not all Statesrequire vehicles to have a front license plate, so ALPRmay have the highest recognition rates using rearplates. Depending on how the system is implemented,data bandwidth may be especially high for ALPR(e.g., if video or camera images are transmitted).CostsThe FHWA Knowledge Resources database providessome cost examples for ALPR. Eberline (2008)provides cost estimates for an ALPR system usedon freeways in Arizona to detect unregistered anduninsured vehicles. He estimates that the camerascost $20,000 each with an approximate installationand supporting hardware cost of $4,000 per camera.Given that one camera is required for each monitoredlane of traffic, actual costs will be higher if multilanecoverage is needed. The Texas TransportationInstitute estimated that ALPR would cost $25,000for a four-lane installation, where each lane has itsown sensor (ITS International, 2010).Rural Data Collection TechnologyFederal Highway Administration14