Question

In 1974, the United States instituted a national speed limit of 55 miles per hour (mph), a move that generated a great deal of controversy. Proponents of the lower speed limit managed to avoid repeal of this national speed limit by effectively arguing that driving at 55 mph significantly reduced the number of traffic fatalities on U.S. highways. The argument was based on the fact that the total number of traffic fatalities dropped from 55,511 in 1973 to only 46,402 in 1974. Because people have questioned the validity of this argument, you are going to examine more rigorously the hypothesis that the reduction in fatalities was due to the institution of the 55 mph speed limit.

Procedure. Since the change to a 55 mph speed limit occurred a number of years ago, you must use archival data in your study. The U. S. government routinely makes available a wide variety of data on the U.S. population. Most public and private libraries either own or would be able to get the national or state statistics you need. Here is the data you would obtain for the present research question:

Table 1: Annual Traffic Fatalities on U.S. Highways

Year Number of fatalities

1966 53,041

1967 52,924

1968 55,200

1969 55,791

1970 54,633

1971 52,660

1972 56,278

1973 55,511

1974 46,402

1975 45,853

1976 47,038

1977 49,510

1978 50,226

Source: U.S. National Center for Health Statistics, Vital Statistics of the United States, annual.

One process of policy implementation decision is the rational comprehensive decision making process. Optimum decisions are the goal. While most of the literature and focus is on economic analysis of optimality, we also need to consider social optimality. Part of that process is through empirical analysis of data and determining its validity. Our knowledge of research designs can be a valuable tool. The purpose of this case analysis is to use those tools.

The hypothesis for this policy implementation analysis is the reduction in fatalities was due to the institution of the 55 mph speed limit. Using about 1000 words (three pages of discussion) and at least 3 scholarly references (one can be the text), review this case and respond to the questions:

What kind of threats to internal validity do these events represent?
Is this policy effective? Does the increasing number of fatalities after 1974 have any implications for the effectiveness of the speed limit intervention?
What is/are one or more of the rival explanations?
What would be at least one social cost of this policy? How is it defined and measured?
What decision making theory was used? What theory should have been used?
As a final paragraph, conclude how this case discussion can assist public administrator's decision making in their role as implementing policy.

Answer 1

One way to interpret these data is simply to graph them and examine the pattern of fatalities across
time. (For future reference, other statistical techniques have been developed specifically for
application to interrupted time series data.

1. Label the abscissa (the horizontal axis) with the years 1966 through 1978, and label the
ordinate (the vertical axis) with the numbers 45 through at least 57. The numbers on the ordinate
refer to thousands of fatalities, and you should note that fact on your graph.
2. Determine the coordinates for each of the data points. The change in national speed limit
occurred in 1974.
3.  starting at the point halfway between the 1973 and 1974 points on the abscissa, draw a
dotted vertical line up to the top of the graph to represent the speed limit intervention.
4. Next, examine the slope of fatalities before the speed limit reduction and compare it to the
slope afterwards. To do this, draw a straight line connecting the 1966 point to the 1973 point. This
line is a rough approximation of the trend in fatalities.
5. You can project, or extend, this straight line to give you an idea of how the trend might have
continued through 1978 if the speed limit had not been changed. Note whether the slope is
increasing or decreasing.
6. To consider the slope after 1973, you can examine two sets of points: first, consider just the
line connecting 1973 to 1974.
7. Next, think about whether there seems to be a line that best describes the points from 1974
to 1978.
8. Note the difference between the slope prior to the intervention and the slope between 1973
and 1974. Note whether there is a difference between the slope prior to the intervention and the
1974 to 1978 slope.
• Decide whether or not, based on your analysis, the reduced speed limit caused a decline in
traffic fatalities. Think about and list other events that could have occurred during 1974 that might
also have contributed to a decline in traffic fatalities (for example, think about reasons for institution
of the 55 mph speed limit in the first place).
•        What kind of threats to internal validity do these events represent?   
•        Regression to the mean is another threat that exists for these data.
•        Explain how it leads to rival explanations for these data.
•        Finally, does the increasing number of fatalities after 1974 have any implications for the effectiveness of the speed limit intervention