Impact of red light cameras on car crashes. Refer to the June 2007 Virginia Department of...

Impact of red light cameras on car crashes. Refer to the June 2007 Virginia Department of Transportation (VDOT) study of a newly adopted photo-red-light enforcement program, Exercise: Impact of red light cameras on car crashes. To combat red-light-running crashes—the phenomenon of a motorist entering an intersection after the traffic signal turns red and causing a crash—many states are adopting photored enforcement programs. In these programs, red light cameras installed at dangerous intersections photograph the license plates of vehicles that run the red light. How effective are photo-red enforcement programs in reducing red-light-running crash incidents at intersections? The Virginia Department of Transportation (VDOT)

conducted a comprehensive study of its newly adopted photored enforcement program and published the results in aJune 2007 report. In one portion of the study, the VDOT provided crash data both before and after installation of red light cameras at several intersections. The data (measured as the number of crashes caused by red light running per intersection per year) for 13 intersections in Fairfax County, Virginia, are given in the table and saved in the REDLIGHT file. Analyze the data for the VDOT. What do you conclude?

Recall that the VDOT provided crash data both before and after installation of red light cameras at several intersections. The data (measured as the number of crashes caused by red light running per intersection per year) for 13 intersections in Fairfax County, Virginia, are reproduced in the next table and saved in the REDLIGHT file. The VDOT wants to determine if the photo-red enforcement program is effective in reducing red-light-running crash incidents at intersections. Use the nonparametric Wilcoxon singed rank test (and the accompanying MINITAB printout, top right) to analyze the data for the VDOT.