A Note on the Use of Passive Alcohol Sensors during Routine Traffic Stops

Abstract

Objectives

To determine whether the use of a passive alcohol sensor (PAS) in routine traffic enforcement increases the driving-under-the-influence (DUI) arrest rate of alcohol-impaired drivers.

Methods

Officers in a Maryland police department were randomly assigned to one of two groups: the first with PAS devices and the second without PAS devices (the control group). Then, the PAS units were switched from the first to the second group. Arrest, PAS, and preliminary breath-test data were collected on 714 nighttime traffic stops over two enforcement periods.

Results

The DUI arrest rate for the officers with and without the PAS was the same, 13%. Officers who made no arrests without the PAS benefited the most from using it. Drivers stopped for an unsafe lane change, driving over the center line, and negligent driving were arrested for DUI 35% of the time.

Conclusions

The PAS appears to increase the DUI arrest rate of officers who rarely make DUI arrests, but it does not increase the DUI arrest rate of officers who normally make DUI arrests without passive sensors. It appears that it could be successful in increasing the overall number of DUI arrests for a police department if issued to, and training is provided to, patrol officers who do not normally make DUI arrests.

BACKGROUND

Drivers in the United States admit to driving within 2 hours of drinking alcohol close to 1 billion times annually (Royal, 2003). Clearly, deterrence to impaired driving in the United States is constrained, as only one driving-under-the-influence (DUI) or driving-while-intoxicated (DWI) arrest is made for every 88 reported episodes of driving over the illegal blood alcohol concentration (BAC) limit (Zador et al., 2000). Deterrence, as described by Ross (1984), is a function of the perceived probability of apprehension, the severity of the resulting sanction, and the swiftness with which the penalty is administered. There is substantial evidence that the most important of those factors is the probability of apprehension because the public is generally unaware of the sanctions for DUI and tend to believe that sanctions can be avoided or ameliorated (Ross, 1992; Ross and Voas, 1989). Thus, raising the perceived probability of apprehension is the most essential element of an effective DUI enforcement program.

The typical DUI arrest proceeds in three steps: (1) the identification and stopping of a vehicle likely to be driven by an impaired driver, (2) an interview with the driver at the vehicle side during which evidence of drinking is collected, and (3) inviting the suspected impaired driver out of the vehicle to conduct standardized field sobriety tests (i.e., performance tests that reveal impairment). The second step is a crucial point in the investigation because the ability to detect an impaired driver over the illegal BAC limit is based on a brief conversation with the driver and behavioral observations. Studies have demonstrated that officers are not very accurate in such assessments (e.g., Moskowitz, 2007). An important cue to drinking used by officers in the field is the odor of alcohol; however, Moskowitz and his colleagues (Moskowitz et al., 1999) demonstrated that the odor of alcohol is not a reliable method for gauging intoxication.

The passive alcohol sensor (PAS) is a device developed to assist police in identifying drinking drivers. The PAS draws in mixed expired and environmental air from 6 inches in front of the subject's face and passes it into a fuel cell sensor that can detect very small amounts of alcohol. The sensor, which is built into the standard police flashlight (see Figure 1), displays the estimated BAC of a driver using a nine LED light bar indicator running from .02 (1 bar) to .12 BAC and higher (9 bars). Evidence from police use of the PAS with drivers stopped at random roadside surveys indicates that using a threshold of 6 out of the 9 light bars can identify 74% of drivers with BACs of .08 or higher with a 99% accuracy (i.e., specificity; Farmer et al., 1999; Voas et al., 2006). Because the PAS measures alcohol in mixed expired air, it is not as accurate as the evidential devices (e.g., the CMI Intoxilyzer 5000 or the Draeger Safety Breathalyzer 900) used to determine the BAC of arrested drivers. The results of these evidential testers are used as evidence in court proceedings. Unlike the evidential devices, however, which are considered a “search” under the Fourth Amendment to the U.S. Constitution and cannot be administered without reason to believe the driver is impaired, the PAS is not considered a “search.” The PAS is considered by many in the enforcement arena as “an extension of the officer's nose”; consequently, the use of a PAS at any point in the investigation is believed to be constitutional (Fields and Henricko, 1986; Manak, 1986).

Figure 1.

Picture of passive alcohol sensor built into a flashlight.

The PAS has been successfully used by officers at sobriety checkpoints where their contact with drivers is generally limited to less than a minute. A number of studies (e.g., Ferguson et al., 1995, 1996; Jones and Lund, 1986; Wells et al., 1997) have demonstrated that officers at checkpoints fail to detect about half of the over-the-limit drivers they interview and that the use of passive alcohol sensors at checkpoints increases the detection of these impaired drivers by approximately 50%. Research conducted by the Insurance Institute for Highway Safety (IIHS) indicates that the PAS units are less effective when used by police on special DUI patrol operations where the drivers have been stopped because they gave an indication of impaired driving and the officer has a longer interview in which to judge their intoxication. In such operations (Lund and Jones, 1987) and on routine patrol (Kiger et al., 1993), they increase detection by about 8 to 10% rather than 50%.

Even though the increase in efficiency produced by the PAS is less for officers on routine patrol, many more officer hours are spent on such patrols than at checkpoints. It is possible that such sensor units could contribute more to the apprehension of impaired drivers if officers assigned to patrol duty used them regularly. To test this theory, we conducted a field trial of the PAS with a Maryland police department using officers on regular traffic patrol operations who had little or no specialized training in apprehending impaired drivers (Fell and Compton, 2007). The objective of this study was to determine whether providing officers on regular nighttime patrols with PAS devices would increase the number of DUI arrests when compared to officers not equipped with the units.

METHODS

The police department randomly assigned officers to two squads. The eight members of Squad A were issued PAS devices and encouraged to use them on every stop from June 2004 to July 2005 (round 1). During that period, the seven members of comparison Squad B continued to conduct traffic patrols without PAS units. At the end of round 1, the PAS units were switched to Squad B, who used them during round 2, which ran from October 2005 to February 2006. During round 2, Squad A returned to normal enforcement procedures without PAS units. Table I illustrates the design and shows the number of nighttime traffic stops made by each squad in each round. Law enforcement officers who did not participate in both rounds were eliminated from analyses. It is probable that more nighttime stops were made by Squad B officers in round 2 because some of these officers changed from the daytime shift in round 1 to the nighttime shift in round 2.

Table I.

Number of nighttime traffic stops

Round 1		Round 2		Total
Squad A (with PAS)	183	Squad A (w/o PAS)	178	361
Squad B (w/o PAS)	131	Squad B (with PAS)	222	353
	314		400	714

PAS = passive alcohol sensor.

Information on each nighttime traffic stop (6 p.m. to 6 a.m.), by each participating officer in both squads, was captured on a data form provided to the research team for analysis. For the squad using the PAS, the officer reported whether the device was used at the stop and, if used, the PAS reading that was recorded. The BAC collected with a preliminary breath-test (PBT) device was also recorded. Finally, if an arrest for DUI was made, it was also recorded on the form. In addition, the reason for the traffic stop and demographic information on the driver were recorded. Any other arrests, citations, warnings, and violations were also recorded, as were the results of an evidential test if performed. The same data were recorded by the squad not using the PAS, except for the data concerning the PAS. At the conclusion of the data collection period, the investigators requested each participating officer to provide information about their general experience with using the PAS.

The data set covered 714 nighttime traffic stops. Chi-square tests (two-tailed) were used to determine whether there were any significant differences in the proportions or rates of interest using the Statistical Package for Social Sciences software (SPSS, 1997). The main effect tested was the DUI arrest rates for officers using the PAS compared to officers not using the PAS.

RESULTS

When both rounds were combined, the PAS was used in 90.4% of the stops made during nighttime patrols by the officers issued the PAS. During nighttime stops, the DUI arrest rate when the PAS was available was 13% (51 arrests in 405 stops), and when it was not available, it was also 13% (39 arrests in 309 stops; see Tables II and III). Overall, the PAS did not increase the DUI apprehension rate for officers on regular patrols.

Table II.

Nighttime DUI arrest rates by PAS use

	Arrests for DUI at night stops (both rounds)
PAS available	51/405 (13%)
PAS not available	39/309 (13%)

DUI = driving under the influence. PAS = passive alcohol sensor.

Table III.

Number of stops equipped with PAS compared to no PAS

	PAS	No PAS	Total
Squad A	183	178	361
Squad B	222	131	353
Total	405	309	714

PAS = passive alcohol sensor.

Examination of the individual records of the officers in Squad A and Squad B indicated that they could be divided into the three groups shown in Table IV, based on their DUI arrests per traffic stop. Seven officers made at least one DUI arrest while they did not have a passive sensor; seven other officers made no DUI arrests when not equipped with the PAS. Finally, one very productive officer made more arrests than all the others combined while without a passive sensor. As shown in Table IV, that very productive officer significantly increased the number of arrests when equipped with a passive sensor. Conversely, the seven officers that made DUI arrests without the sensor actually arrested fewer impaired drivers when equipped with the PAS. In contrast, the seven officers who made no arrests when not equipped with the PAS did make significantly more DUI arrests when the unit was available to them.

Table IV.

Use of the PAS and DUI arrests as a function of officer arrest activity with and without the PAS

Officer group	Without PAS			With PAS			Sig.
	Stops	DUI arrests	%	Stops	DUI arrests	%
7 Officers who made DUI arrests w/o PAS (minus productive officer)	207	19	9	206	13	6	Not significant, p = .276
1 Very productive officer	34	20	59	30	30	100	Significant, p < .001
7 Officers who made no DUI arrests w/o PAS	68	0	0	169	8	5	Significant, p < .004

DUI = driving under the influence. PAS = passive alcohol sensor.

The data collected on these 714 nighttime traffic stops also allowed us to determine whether there were certain reasons for the traffic stops that indicated the potential for impaired driving, as shown by positive readings of the PAS. Table V a shows that three reasons for making a traffic stop—unsafe lane change, failure to drive right of center, and negligent driving (such as weaving)—are strong indicators that drivers are impaired by alcohol. Of the 70 traffic stops made for these three reasons (called “unsafe maneuvers” in Table V), 31% resulted in a positive PAS reading during stops when the PAS was available. This compares to 7% positive PAS readings for stops involving speeding and 17% positive PAS readings for all other reasons for stops (combined). These differences were statistically significant (p < .01).

The same three reasons for traffic stops resulted in significantly more DUI arrests than other reasons. Table VI indicates that, of the 70 stops made for “unsafe maneuvers” (i.e., unsafe lane change, failure to drive right of center, and negligent driving) when the PAS was available, 37% resulted in a DUI arrest compared to 4% of those stopped for the speeding and 13% for all other reasons combined. This was also statistically significant (p < .01). When examining all 714 traffic stops (with and without the PAS), the “unsafe maneuver” traffic stops resulted in a 35% DUI arrest rate, speeding resulted in only a 3% DUI arrest rate, and all other reasons resulted in a 13% DUI arrest rate. These differences were also statistically significant (p < .01).

Table VI.

Reasons for traffic stops in relation to a DUI arrest

	DUI Arrest
Reason for stop	Yes*	No
Unsafe maneuver (unsafe lane change/failure to drive right of center line/negligent driving)	26 (37%)	44 (63%)
Speeding	7 (4%)	190 (96%)
All others (e.g., following too close, failure to yield)	18 (13%)	120 (87%)

^*Yes (p < .01). DUI = driving under the influence.

When the PAS was available during 405 stops, 33 drivers were given a PBT, all with positive BAC results (Table VII). In 28 of those instances, the PAS was used. In 26 (93%) of those instances when the PAS was used, it detected alcohol in the drivers. In only 2 instances (7%) did the PAS fail to detect the 28 drinking drivers confirmed by the PBT result. Even when the PBT result was less than .08 BAC (the illegal limit in every state), the PAS detected alcohol in 4 of 5 instances (80%).

Table VII.

Reliability of the PAS in detecting drivers with positive PBT results

PAS Result	PBT Result
	.02−.079	.08−.159	.16+
Positive reading	4 (80%)	12 (92%)	10 (100%)
Zero reading	1 (20%)	1 (8%)	0 (0%)
Totals	5 (100%)	13 (100%)	10 (100%)

PBT = preliminary breath test. PAS = passive alcohol sensor.

The results of the questionnaire that was administered to the participating officers after the study supported the data presented in Tables I-IV. Five of the seven officers who did not make DUI arrests without the PAS indicated that it helped them detect impaired drivers when they used it, whereas only two of the six responding officers who did make DUI arrests without the PAS indicated that the PAS helped detect impaired drivers. The main complaint about using the PAS at traffic stops was the requirement to hold it in close proximity (∼6 inches) to the driver's face.

DISCUSSION

Although the PAS detected 93% of drivers with positive PBT results, giving an indication of its validity, the overall DUI arrest rate during nighttime stops with the PAS was only 13%, the same as without the PAS. The PAS appeared to help the officers who typically do not make DUI arrests. The PAS produced a positive result for 22 of the 23 cases in which the PBT BAC was over the legal .08 limit, indicating its utility for detecting impaired drivers. These results must be interpreted with caution because of the small number of officers involved, the substantial variance between officers in arrest activity, and the limited number of stops and arrests. Also, the officers did not conduct a PBT or a PAS test on every motorist stopped, so the prevalence of high BAC drivers in the population at risk is unknown.

Past studies have shown that police officers do not like using the PAS at traffic stops (Leaf et al., 1996). This is reflected in the fact that although the PAS unit has been available for more than 20 years, the penetration rate into the police community is very low. The devices have been substantially improved in response to police complaints. Yet only about 4,000 PAS units have been sold to police departments in the past 5 years, and only 8,000 in the past 10 years. Currently, an optimistic estimate of use would be that about 2,000 of 500,000 traffic law enforcement officers around the country are using the PAS (about a 0.4% penetration rate) (personal communication with J.R. Kelsey, PAS Systems International).

The use of the PAS is probably most beneficial at sobriety checkpoints when police typically only have less than a minute to determine whether a driver is impaired by alcohol. Although moderate social drinkers usually exhibit behavioral signs of intoxication at BACs exceeding .08, many alcoholics and problem drinkers show no outward signs such as slurring their words or fumbling with their driver's licenses. Thus, it is important for police to use the PAS at checkpoint operations. With current congressional support and funding for highly publicized national sobriety checkpoint blitzes at least twice a year, the use of the PAS by police during these national crackdown periods (Labor Day week and the Christmas–New Year's holiday period) could serve as a general deterrent to impaired driving (Fell et al., 2004). Additionally, PAS units could increase the detection and arrest of impaired drivers at these checkpoints. The PAS may also be beneficial in the enforcement of zero-tolerance laws (no alcohol) for drivers younger than 21. Further, passive sensors can potentially increase the arrest efficiency of officers who make few DUI arrests and stimulate the checking of vehicles during nighttime traffic patrols. The intriguing case of the single highly productive officer who increased his citations with the PAS opens the possibility that officers who make large numbers of arrests may also profit from the use of the PAS, but clearly confirmation of these preliminary results is required.

Table V.

Reasons for traffic Stops in relation PAS reading

	PAS Results
Reason for stop	Positive*	Negative	Missing
Unsafe maneuvers (unsafe lane change/failure to drive right of center line/negligent driving)	22 (31%)	35 (50%)	13 (19%)
Speeding	13 (7%)	164 (83%)	20 (10%)
All others (e.g., following too close, failing to yield)	23 (17%)	104 (75%)	11 (8%)

^*Positive (p < .01). PAS = passive alcohol sensor.