2007 National Roadside Survey of Alcohol and Drug Use by Drivers

2007 National Roadside Survey of Alcohol and Drug Use by Drivers: Drug ResultsMethodsThis section of the report briefly summarizes the methodology used in conducting the 2007 NRS,with special emphasis on sampling procedures. A separate Methodology Report (Lacey et al.,2009a) provides detailed descriptions of the multiple components of the data collection process.Survey Sampling ProceduresThis section presents an abridged description of the sampling approach we followed inconducting the 2007 NRS. Because it is infeasible to conduct surveys on all the roads in theUnited States, we constructed a sampling system for the 2007 NRS that represented the wholeNation but required interviewing only a practical portion of the almost 203 million drivers onU.S. roads (FHWA, 2006). As in the three prior surveys, the area covered in this study waslimited to the 48 contiguous States.For practicality, we limited locations to roadways where surveys could be performed safely andwith sufficient traffic to recruit the number of participants required for valid estimates of thenational prevalence of drinking and drug involved-drivers, as did prior NRS studies.The past three national surveys provided information on private four-wheel vehicle operators atrepresentative, then randomly selected, locations during weekend, nighttime periods whendrinking and driving is most prevalent. The 2007 NRS covered the same time periods and addedtwo Friday daytime periods. As in the three earlier surveys, the 2007 NRS excluded commercialvehicles but, unlike previous practice, included motorcycles.The 2007 NRS followed the practice of the 1973, 1986, and 1996 national surveys by using amultistage sampling system that represented the drivers at risk for crash involvement in the 48contiguous States. In this process, the initial sample structure was taken from NHTSA’s NationalAutomotive Sampling System/General Estimates System (NASS/GES) (NHTSA, 1995), whichwas constructed to provide a basis for making nationally representative estimates of highwaycrashes. The four steps included:1. Selecting the primary sampling units (PSUs), which are cities, large counties, or groupsof counties from within four regions of the United States and three levels of populationdensity.2. Randomly selecting 30 specific square-mile-grid areas within each PSU, and randomlynumbering them to form an order of priority from among the total of all the square milesectors comprising the PSU area. Then we attempted to recruit the cooperation of locallaw enforcement agencies that had jurisdiction over the selected grids. One lawenforcement agency often would cover several of the selected square mile areas.3. Identifying appropriate survey sites within the square-mile-grid areas. Appropriate siteshad a safe area large enough to accommodate the survey operation and had sufficienttraffic flow to generate an adequate number of subjects. In some cases, more than onesuch location was available within a square mile grid. In this case, the survey managerexercised her/his judgment to select the optimal location for safe data collection. Thisresulted in selection of five data collection or survey sites within each PSU.17