Are U.S. traffic fatalities related to the speed limit and/or road type? Consider the data found in the file P3_74.XLS.

a. Do the average number and/or variability in the number of traffic fatalities occurring on interstate highways tend to increase as the speed limit is raised above 55 miles per hour? Explain your answer. 

b. Do the average number and/or variability in the number of traffic fatalities occurring on non-interstate roads tend to increase as the speed limit rises above 35 miles per hour? Explain your answer.

c. Do the average number and/or variability in the number of traffic fatalities occurring on a road with a posted speed limit of 55 miles per hour tend to change with the road type (..e., interstate versus non-interstate highway) Explain your answer. 

d. Based on these data, which combination of speed limit and road type appears to be most lethal for U.S. drivers? 

e. Based on these data, which combination of speed limit and road type appears to be safest for U.S. drivers?