Traffic fatalities and 9/11. Figure 0.4 provides information on the number of fatal traffic accidents by month for the years 19962001. 8 The vertical blue line above each month gives the lowest to highest number of fatal crashes for that month for the years 19962000. For example, in January the number of fatal crashes for the five years from 1996 through 2000 was between about 2600 and 2900. The blue dots give the number of fatal crashes for each month in 2001. The numbers of fatal crashes from January through August of 2001 follow the general pattern for the five preceding years as we see the blue dots are well within the blue lines for each month.

(a) What happened in the last three months of 2001? The number of fatal crashes in October through December of 2001 are consistently at or above the values for the previous five years. How can you tell this from the graph?

(b) On September 11, 2001, terrorists hijacked four U.S. airplanes and used them to strike various targets on the East Coast. In part (a), we saw from the graph that fatal traffic accidents seemed to be unusually high in the three months following the attacks. Did the terrorists cause fatal crashes to increase? Can you give a simple explanation for the apparent increase in fatal crashes during these months?