Question

Impact of red-light cameras on car crashes. To combat red-light-running crashes, many states have installed red light cameras at dangerous intersections to photograph the license plates of vehicles that run the red light. How effective are photo-red enforcement programs in reducing redlight-running crash incidents at intersections? The Virginia Department of Transportation (VDOT) conducted a comprehensive study of its photo-red enforcement program. In one portion of the study, the VDOT provided crash data before and after installing 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 Fairfax County, Virginia, intersections are given in the table. Analyze the data for the VDOT. What do you conclude?

Source: Based on “Research Report: The Impact of Red Light Cameras (Photo-Red Enforcement) on Crashes in Virginia” from Virginia Transportation Research Council.

Short answer

$\text{Test statistic}\left(\text{t}\right)\text{is 0.8327}$

Step-by-step solution

Red-light running crash

Red light cameras are an excellent deterrent to red light running. The most significant approach to convince individuals to follow any rule is via enforcement, yet it is impractical for police to be at every crossing. Cameras can help to fill the hole. According to IIHS research, cameras lowered the fatal red light running collision rate in big cities by 21% and a 14 percent reduction in all types of fatal collisions at signalized junctions.

Analyse the data

$\begin{array}{rcl}\overline{d}& =& \frac{13.09}{13}\\ & =& 1.007\end{array}$

$$\begin{gathered} Sd=\sqrt{\frac{1}{\left(n-1\right)}\left(n\sum d{i}^2-{\left(\sum {}_{i}di\right)}^2\right)} \\ =\sqrt{\frac{1}{12}}\left(13\times 30.7281\right)-{\left(13.09\right)}^2 \\ =4.360019 \end{gathered}$$

$$\begin{gathered} \text{Test statistic}\left(\text{t}\right)\text{=}\frac{\overline{d}}{{\displaystyle \raisebox{1ex}{$S_d$}\!\left/ \!\raisebox{-1ex}{$\sqrt{n}$}\right.}} \\ =\frac{1.007}{\frac{\left(4.360019\right)}{\sqrt{13}}} \\ =0.8327 \end{gathered}$$

Conclusion

There is no compelling proof that VDOT Crashes do not affect camera placement (not reduced significantly).