2007 National Roadside Survey of Alcohol and Drug Use by Drivers

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2007 National Roadside Survey of Alcohol and Drug Use by Drivers: Drug Results4. Selecting at random drivers to be interviewed from the traffic passing by the survey site.The total number of eligible vehicles was counted to determine the proportion of thetraffic passing by each survey site that was sampled.These sampling procedures were followed to ensure that the probability of selecting a PSU, asurvey location within the PSU, and a driver at a survey location was known at each of thesample design stages. Knowing these probabilities permitted the computation of the probabilitythat a given driver would be interviewed in the survey. This was done by multiplying thesampling probabilities at each of the four steps to obtain the final overall probability of beingsampled. The weight given to each case in the final totals (sampling weight) was computed asthe inverse of the sampling probability. This statistical procedure accounts for differences amongPSUs in the size of the driver population. In other words, although we sampled approximatelythe same number of drivers at each PSU, the actual number of individuals driving at eachsampling site was not uniform. To make the sample of drivers at each site representative of theactual number of drivers, we applied the above-described weights. As a result, driversinterviewed at sites with a relatively heavy traffic flow (i.e., a relatively large pool of actualdrivers) carry a larger weight than drivers sampled from sites with less traffic loads. This ensuredthat the basic requirement of sampling theory—that every driver has an equal chance of beinginterviewed—was met by adjusting for the biases inherent in the selection of locations within thesampling frame.The major barrier to carrying out this staged sampling system was obtaining law enforcementsupport for the survey. In some localities, city attorneys or law enforcement leadership withconcerns such as potential liability and scare resources declined to participate. In these cases,substitution PSUs were obtained. Although this process was time-consuming, similar difficultieshad also been experienced in all three previous NRS studies. Replacement PSUs were chosenfrom within the same GES geographic region 9 and the same GES PSU type (city, large suburbanarea, all others) as the unavailable PSU. For more information on PSU replacement, see Lacey etal. (2009a). The 60 PSUs used in the 2007 NRS are shown in Figure 1.9 GES defines four geographic strata.18
2007 National Roadside Survey of Alcohol and Drug Use by Drivers: Drug ResultsFigure 1. Map of Sixty 2007 National Roadside Survey SitesAs mentioned, the roadside survey procedures used in the 2007 NRS followed, as closely aspossible, those used in the previous three surveys (see Lacey et al., 2009a; Lestina et al., 1999).However, the 2007 NRS departed from the earlier surveys in several important ways. The earliersurveys included only a brief questionnaire and a breath test that generally required less than 5minutes of a participant’s time. The 2007 NRS included a more extensive set of questions (basesurvey, a drug questionnaire, questions about interaction with the criminal justice and treatmentsystems, an alcohol-use disorder and a drug-use disorder survey). The 2007 survey protocol alsoattempted to collect two biological samples (oral fluid and blood) from participants, as well as abreath test. Data collection for the earlier surveys was conducted by three teams of threeinterviewers; the 2007 NRS consisted of 6 teams of 10 to 12 members. The earlier surveys wereconducted at 24 PSUs, whereas the 2007 survey was conducted at 60 PSUs. This increase in thenumber of PSUs allowed us to maximize the use of all possible PSUs defined by the NASS/GESand increase the representativeness of the sample. The earlier surveys had four 2-hour datacollection periods on weekend nights; the 2007 survey added a 2-hour data collection periodduring the daytime on Fridays, for a total of five 2-hour survey periods during the weekend.Finally, the number of participants in the 2007 survey was about three times as many as in the1973 study.PIRE employed and trained six specialized teams of interviewers from both the east and westcoasts of the United States. All staff was trained during the summer of 2007. Surveys began theweekend of July 20 and 21, 2007, and concluded 20 weeks later on December 1, 2007. As in thethree previous NRS studies, nighttime surveys were conducted between 10 p.m. and midnight,and between 1 a.m. and 3 a.m. on both Friday and Saturday. For the 2007 survey, a 2-hourFriday daytime data collection period was added, either between 9:30 a.m. and 11:30 a.m. orbetween 1:30 p.m. and 3:30 p.m. The daytime data collection period was randomly selected foreach PSU. The daytime periods were added to determine the extent of alcohol- and drug-19
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DOT HS 811 249December 2009
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