Questions 17-20 are based on the following passage.

    This report presents the results of a study conducted for the National Highway Traffic Safety Administration (NHTSA) to assess the effects of two programs that were implemented to reduce the incidence of aggressive driving. The programs were conducted by the Marion County Traffic Safety Partnership (Indianapolis, Indiana), and The Tucson, Arizona, Police Department.

    Samples of vehicle speed, collected unobtrusively inthe special enforcement zones, and crash incidence served as the primary measures of the programs’ effect. The total number of crashes in the Marion County special enforcement zones increased by 32 percent, compared to the same six-month period one year earlier; the number of those crashes with primary collision factors (PCFs) associated with aggressive driving increased by 41 percent. That is, the total number of crashes increased, but the crashes with aggressive driving PCFs increased at a greater rate. The change in proportion of crashes with the target PCFs provides a better measure than crash frequency because it eliminates the effects of changes in traffic volume and other factors that might have contributed to the overall increase in crash incidence.

     In this regard, the Marion County zones experienced a six percent increase in the proportion of all crashes with aggressive driving PCFs, despite the extensive publicity and special enforcement efforts. The number of crashes in Tucson’s special enforcement zones increased by ten percent, but the number of crashes with aggressive driving PCFs increased by less than one percent. More importantly, the proportion of all crashes with target PCFs decreased by eight percent. Thatis, crash incidence increased overall in Tucson's zones, but the proportion of those crashes with aggressive driving PCFs declined.

    The original purpose of this report was to present, rather than compare, the two aggressive driving programs. However, comparisons are inevitable and the substantially different results of the two programs require an explanation. First, it is important to understand that it is impossible to control all of the variables that could influence the outcome of study when conducting large-scale quasi-experiments, such as the programs described in this report. Highway maintenance projects and large residential developments that increase traffic volumes on surface streets are examples of unexpected and uncontrolled variables that can affect dependent measures in a field study involving, driver behavior. Every attempt was made to identify and control relevant variables within each program, but the research was not originally designed to support systematic comparisons between the two programs. Marion County, Indiana, and Tucson, Arizona, are different in many ways, and it is possible that some of those differences could be responsible for the differential results reported here.


What is the primary purpose of this passage?