DEVELOPMENT OF AUTOMATED PROCEDURES FOR ASSESSING THE BEHAVIOR OF DRIVERS APPROACHING CHECKPOINTS

The difference between sobriety checkpoints and roving DWI patrols is analogous to the difference between trapping and hunting strategies among commercial fishermen. For example, lobster fishermen, crab trappers, and most gillnetters deploy their gear in locations known to be inhabited by the target species, in much the same way that checkpoints are set up at locations known for DWI arrests or alcohol-involved crashes. In contrast, some fishermen adopt a hunting strategy by searching for indicators of fish by both visual and technical means, then pursuing their prey, in the same manner that roving FIGURE 6 Number of DWI arrests and vehicle contacts in four sobriety

checkpoint programs (based on data from Stuster and Blowers, 1995).

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patrol officers search for, then stop, motorists who exhibit DWI cues. The trapping strategy is fundamentally passive and dependent upon the appearance of targets in the area; the hunting strategy is not. Hunters can increase their catch by increasing their effort with the same amount of equipment (e.g., spending more time “on the grounds,”

prospecting new areas); the means for trappers to increase their catch is to increase the amount of gear deployed. If we continue the analogy, roving patrols can increase their DWI arrest rate by improving the efficiency of their effort (e.g., spending less time completing paperwork and more time on the road “prospecting” new areas for impaired drivers, such as the out-of-the-way locations of underage drinking). In contrast, checkpoint programs rarely experience increasing arrest rates; a declining arrest rate is a measure of a checkpoint program’s deterrence on drivers. However, to be effective deterrents, checkpoints must be perceived by the public to substantially increase the probability of detection and arrest for those driving while impaired.

Several factors can contribute to some alcohol-impaired drivers passing

undetected through sobriety checkpoints, including high officer workloads and increasing reluctance by officers to inhale the breath of hundreds of motorists. Automated DWI detection might contribute to public perceptions of arrest risk and materially assist

officers, especially if sensors can detect subtle patterns of vehicle movements that are not readily apparent to human observers. For these reasons, NHTSA is sponsoring research to explore the possibility of automatically detecting impaired drivers in the approach lanes of sobriety checkpoints.

Research conducted under this contract has identified the vehicle movements characteristic of alcohol-impaired driving and a technology that promises the accuracy necessary to measure those movements (i.e., lateral displacement, speed and braking).

The system that presently is under development has at its heart a laser speed gun, similar to those used by law enforcement agencies across the United States. Instead of remaining focused on a target vehicle, however, the laser’s beam sweeps across the lane from left to right, then back, firing 90 times during each scan (45 times per sweep). A vehicle reflects the infrared pulses when it enters the field of view of the scanning laser (beginning at a range of about 300 feet); the ranges to the vehicle and the associated angles (from the laser to the vehicle) are obtained from these optical returns as the laser sweeps across the lane. Because all the ranges and angles are known, the distance of the vehicle’s edge from the lane line can be calculated twice each scan. The scanning laser sweeps across the lane and back approximately three times each second. The prototype scanning laser has been designed to be located at the side of the road (on a low tripod); a laptop computer is connected by cable to perform system calibration and data acquisition.

The next step in this research and development project will be to conduct a controlled field test using dosed drivers at a simulated checkpoint. The purpose of the controlled field test will be to determine if drivers exhibit measurable driving behavior that can be correlated with BAC level. Analyses will include vehicle movement variables of (1) lateral displacement (lateral position, and the frequency and amplitude of excursions), and (2) vehicle speed (speed at acquisition, speed at points within the approach lane, and changes in speed). Analyses will attempt to identify vehicle movements and patterns of variables that correlate with BACs =0.08 percent, and if possible, lower BAC levels. In this regard, the scanning laser system is distinguished from other efforts to detect DWI using

advanced technology by directly assessing driving performance, rather than searching for other, indirect measures of possible DWI.

The test plan has been approved by NHTSA’s Human Use Review Panel and the controlled field test will be conducted in October 1998. If meaningful correlations are found during the controlled field test, the ultimate product of this research program will be an automated system to assist officers in screening alcohol-impaired drivers at sobriety checkpoints. The ultimate system might be slightly larger than a conventional laser speed gun and incorporate real-time processing and decision-aiding functions.

CONCLUSIONS

Operating a motor vehicle involves the performance of divided-attention tasks for which humans are poorly suited. A driver must attend to several sources of information, using all sensory modalities. The driver must process the information, make a continual series of decisions, and translate those decisions into control adjustments, primarily involving speed and direction. Driving errors are common under optimum conditions, and even minor errors and misjudgments can be fatal. The probability of error is greatly increased when driver performance is impaired by alcohol.

An alcohol-impaired driver can exhibit impairment to on-the-road observers in several ways. Specifically, judgment can be impaired, causing a driver to follow another vehicle too closely, make an unsafe lane change, or perform other inappropriate acts. A driver’s ability to process information can be impaired, resulting in speed and braking problems, and problems associated with degraded vigilance. Impairment also is evident in problems maintaining proper lane position, for example, weaving or turning with a wide radius.

Focusing on the tasks performed by drivers on the road has permitted us to identify 24 visual cues that are highly predictive of DWI. However, the visual cues available to detect impaired drivers as they approach a sobriety checkpoint are limited by the constrained driving conditions to vehicle speed, change in speed, vehicle lateral displacement, and change in lateral displacement. Because these vehicle movement variables are among the most predictive on-the-road DWI cues, it is possible they will emerge as useful measures of driving impairment when observed unobtrusively by an automated decision-aiding system.

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