Statistical Analysis of Red Light Cameras in Texas Intersections
Over 30,000 fatal crashes occur in the United States each year (NHTSA 2009). Many of these occur in intersections, specifically when drivers fail to stop at a red light. Several cities now use red light cameras to automatically give tickets to those who run red lights. These cameras identify the license plate numbers of the offending vehicles and mail tickets to the registered drivers of the cars. The stated use of these cameras is to improve public safety and decrease the number of fatal crashes by deterring motorists from running red lights and thus causing accidents; however, critics – including AAA - think that the cameras are installed merely to generate increased revenue for local and state governments (Batista 2010). For this project, we will set out to determine if red light cameras actually decrease the amount of crashes and decrease the likelihood of injury for crashes that occur in an intersection.
For this application project, we intend to use data gathered by the Texas Department of Transportation (2011) regarding the number of accidents at specific intersections before and after the installation of red light cameras at those intersections. The population is all intersections with red lights, and the sample is chosen intersections in Texas. We assume that all motorists are informed of the presence of the red light cameras at each of the specified intersections, that each motorist who runs a red light is captured by the camera and given a ticket, and that no other change affects the intersection except the installation of red light cameras. Once the data is collected, we will block for the crash type: fatal, injury, and non-injury. We will compute the percent change in the number of each type of accident before and after installation of the red light cameras at each intersection. We will also look at the percent change in the total number of crashes before and after installation of the red light cameras at each intersection.
Assuming that the percent changes at each intersection form a normal distribution for each block, we can then conduct a hypothesis test for each block (fatal, injury, non-injury, total) with the null hypotheses being that X = 0, where X represents the percent change in the number of crashes before and after the installation of the red light cameras. The alternate hypotheses will be that X < 0, meaning that the number of accidents in an intersection has decreased following the addition of red light cameras.
We shall examine the possible Type I and Type II errors for these hypothesis tests. Type I error would occur when there is no percent change in the number of crashes before and after the red light cameras are added, but we reject the null hypothesis anyway. This type of error could lead local and state governments to install the cameras thinking that they effectively reduce the number of crashes when in fact, they have no effect on the accident rate. Type II error would be that we do not reject the null hypothesis when in fact there is a decrease in the number of crashes. This type of error would result in governments choosing not to install red light cameras when they could reduce the number of accidents and help save lives. 95% confidence intervals for the mean percent change in accidents for each block will be computed and examined.
The results of this statistical analysis could demonstrate the effectiveness of red light cameras and the validity of government’s arguing that they improve safety, disproving the theory that they are merely installed to increase revenue. If a larger amount of data was collected from across the country, further statistical analysis could determine whether or not red light cameras actually are effective in helping to decrease the number of crashes and thus save lives.
Batista, Elysa. (2010, May 13). Crist signs Fla. bill legalizing red light cameras. Naples Daily News. Retrieved from http://www.naplesnews.com/news/2010/may/13/crist-signs-fla-bill-legalizing-red-light-cameras/
National Highway Traffic Safety Administration (NHTSA). (2009). Fatality Analysis Reporting System (FARS) Encyclopedia [data file]. Retrieved from http://www-fars.nhtsa.dot.gov/Main/index.aspx
Texas Department of Transportation. (2011). Red Light Cameras – Annual Data Reports [data file]. Retrieved from http://www.txdot.gov/safety/red_light_reports.htm