The data presented in the previous section of this report show that more crashes occurred during the program periods in both Marion County and Tucson than in the same six-month periods of the previous year, when no special aggressive driving enforcement was conducted in either community. Marion County’s special enforcement zones experienced 32 percent more crashes than during the same period of the Year 2000; crashes in Tucson’s special enforcement zones increased by ten percent, overall. Further, in Marion County, crashes with primary collision factors associated with aggressive driving increased by 21 percent in the comparison zones and by 41 percent in the special enforcement zones. Based on increases such as these, it would be reasonable to question whether special enforcement has a stimulating, rather than an inhibiting, effect on the incidence of aggressive driving. One interpretation might be that the special enforcement and PI&E programs influenced some motorists to drive more slowly, resulting in increased opportunities for aggressive drivers, whose behavior is less likely to be influenced by the programs, to overtake the slower drivers, with the encounters leading to the increase in crashes.
The Introduction to this report discussed the apparent increase in road rage and aggressive driving, and attributed the increase in observed incidents, largely, to the 27 percent increase in vehicle miles traveled per year that occurred in the U.S. during the final decade of the Twentieth Century. With the addition of only one percent of roadway during that period, one might expect, overall, 26 percent increases in all traffic-related phenomena, or increases of approximately two and a half percent each year in the absence of mitigating factors. However, the relationship between traffic volume and phenomena such as crashes may not be linear, nor are increases in traffic volume uniformly distributed throughout the network of roads and highways. That is, a small increase in traffic volume from one year to the next might result in a disproportionate change in crash incidence. Further, changes in traffic volumes are of greater magnitude on some roadways than on others. Perhaps factors such as these influenced the results of the aggressive driving programs conducted by the Marion County Traffic Safety Partnership and the Tucson Police Department.
Data were obtained from the Department of Public Works of the
City of Indianapolis, and the Department of Transportation of the
City of Tucson, to determine if any changes in traffic volumes were
measured that might help explain the increases in crashes that occurred
during the program periods. Traffic counts are not performed every
year nor in all locations within a community. For these reasons,
in some cases extrapolation was required from 1999 data and in others
from sections of roadway adjacent to a special enforcement zone.
Table 7 presents the estimated changes in traffic volumes in the
Marion County and Tucson special enforcement zones from the Year
2000 to 2001, based on the traffic count data provided by the cities.
The table shows that Marion County’s zones experienced estimated
increases in traffic volumes ranging from five to ten percent, with
an average estimated increase of eight percent. The estimated traffic
volume increases in Tucson’s zones ranged from five to 18
percent, with an average estimated increase of nine percent. The
actual and estimated traffic count data show that the special enforcement
zones of both aggressive driving programs experienced increases
in traffic volumes substantially greater than the national average
of two and a half percent between the Year 2000 and 2001.
The estimated increases in traffic volumes between the years 2000 and 2001 provide a partial explanation for the increases in crashes in the special enforcement zones of both communities during their aggressive driving programs. The estimated nine percent increase in traffic volume in Tucson’s zones was accompanied by a ten percent increase in crashes. An increase in crash incidence comparable to the increase in traffic volume is a reasonable expectation. The data from Marion County, however, are not as easily explained.
It was estimated that traffic volume increased by an average of
eight percent in Marion County’s special enforcement zones,
while the number of crashes increased 32 percent during the aggressive
driving program, compared to the number of crashes during the same
six-month period one year earlier. That is, crashes increased at
a rate four times the increase in traffic volume. The most likely
explanation for this difference is that the estimates of traffic
volume are inaccurate; in particular, the extrapolations from previous
years and neighboring zones were statistical projections that did
not take into account any unmeasured increases in traffic volume
resulting from the highway maintenance and construction projects
that were mentioned by officers as a possible explanation for the
increase in crashes.
Table 8 presents a summary of the relevant crash data for both Marion County and Tucson. The table shows that the total number of crashes in the Marion County special enforcement zones increased by 32 percent, and the number of those crashes with aggressive driving PCFs increased by 41 percent, as reported previously. That is, the total number of crashes increased substantially, but the crashes with aggressive driving PCFs increased at a greater rate. Crashes with the target PCFs is a more relevant metric than total crashes, but change in the proportion of crashes with the target PCFs provides the best overall measure of program effect because it eliminates the influence of differential traffic volume and other uncontrolled factors that might contribute to an overall increase in crash incidence. In this regard, Table 8 shows that the Marion County zones experienced a six percent increase in the proportion of all crashes with aggressive driving PCFs; that is, total crashes increased by 31 percent, crashes with aggressive driving PCFs increased by 41 percent, but the proportion of total crashes with aggressive driving PCFs increased by six percent.
Greater crash incidence in the enforcement zones could be expected because those locations were selected on the basis of disproportionate aggressive driving. Enforcement and comparison zones experienced comparable numbers of crashes during the preceding year, but the locations of greatest concern to the officers in both communities were selected to be the enforcement zones. Table 8 also shows that total crashes and crashes with the target PCFs increased in the Marion County comparison zones (24 and 21 percent, respectively), but the increases were not as great as in Marion County’s enforcement zones. The proportion of all crashes with the target PCFs declined by one percent in the comparison zones and increased by six percent in the zones in which Marion County’s special enforcement was conducted. Tucson’s comparison zones experienced two percent fewer crashes during the program, compared to the same six-month period one year earlier, while crashes in Tucson’s enforcement zones increased by ten percent. However, the proportion of all crashes with the target PCFs declined by three percent in Tucson’s comparison zones and by eight percent in the zones that received special enforcement.
The data summarized in Table 5 showed that Marion County’s 32 percent overall increase in crashes is composed of a 39 percent increase in property damage only (PDO) crashes and a 17 percent increase in injury crashes. In contrast, Tucson’s ten percent overall increase in crashes is composed of a 20 percent increase in PDO crashes and a three percent decrease in injury crashes. Higher vehicle speed is a more likely contributing factor to injury crashes than to PDO crashes. Injury crashes increased in Marion County’s special enforcement zones, but declined in Tucson’s zones, which suggests greater effectiveness of Tucson’s countermeasure program. This interpretation of the crash data is supported by the results of the speed samples presented in Tables 3 and 4. Average speeds declined slightly in Marion County, and at a greater rate in Tucson, possibly in response to special enforcement and other programmatic efforts. 10
Despite the differences between Marion County and Tucson, comparisons can be made if it is assumed that any unknown differences are unlikely to influence the incidence of crashes or average vehicle speed, and that known differences have been identified and controlled. For example, a hypothetical difference, such as driving style, or a known difference, such as climate, are rendered irrelevant by comparing percent changes from baseline conditions at each location, rather than comparing actual crash frequencies; this procedure permits comparisons of zones with different baseline crash frequencies within a program, and comparisons of programs. Also, we calculated the proportions of crashes with aggressive driving PCFs to facilitate comparisons by eliminating the confounding effects of increased numbers of crashes, overall. These methods provide reasonable assurance that the only relevant independent variables are the components of the two countermeasure programs.
If the assumptions concerning comparability are accepted, the data presented in this report show that Tucson’s aggressive driving program achieved the intended result of reducing the incidence of aggressive driving, while the Marion County program did not. As described previously, each program received a grant of $200,000, but the managers of the Tucson program devoted considerably more of their resources to enforcement effort than the Marion County managers, and considerably less to conventional paid advertising to publicize their program. Further, the Tucson program assigned two officers full-time and two officers part-time to conduct the special enforcement patrols and deployed the officers nearly every day of the six-month program period. In contrast, special enforcement in the Marion County program was conducted as overtime duty by 42 officers from six different law enforcement agencies, with teams of five officers deploying, on average, one day in three.
This comparison of the Marion County and Tucson aggressive driving programs suggests that limited resources might be better spent on officer labor than on publicity. It also appears that focusing enforcement responsibility on a small team assigned full-time to the special enforcement patrols is probably more effective than sharing the responsibility among a large number of officers as occasional overtime duty.
THE PROGRAMS MORE EFFECTIVE?
Both programs were well planned and conducted by skilled and highly-motivated professionals. It is quite likely that at least Tucson’s countermeasure program achieved as much success as reasonably can be expected based on the actual, rather than the news media-inflated, magnitude of the “aggressive driving problem.” Further, Tucson’s eight percent decline in the proportion of crashes with aggressive driving PCFs is a considerable accomplishment. The eight percent decline represents approximately 13 fewer crashes in the special enforcement zones during the six-month program period than would have occurred had the program not been conducted (i.e., five injury crashes and eight property damage only crashes). Some portion, if not all of those prevented crashes, could be attributable to the Tucson program. Although statistical tests were not conducted, all measures are consistent with the hypothesis that a program that combines special enforcement with publicity about the enforcement can change driver behavior. Economists have calculated the overall costs resulting from crashes of different severity. The savings to society from preventing eight PDO crashes and five crashes with only moderate injuries to one person per crash greatly exceeds the cost of Tucson’s aggressive driving program.
Based on the experiences of the current study, it is recommended that observational methods also be used in future evaluations of aggressive driving countermeasure programs. In particular, trained data collectors, positioned unobtrusively at roadside or at intersections, could record observed incidents of aggressive driving on paper or electronic forms. The observers would have the advantage of actually seeing the drivers and the surrounding traffic and, thus, be able to discriminate, in most cases, whether the behavior was intentional or the result of human error or misjudgment. Observations conducted according to a systematic sampling plan, involving time of day, day of week, and location, could provide robust data sets for comparing baseline to treatment conditions. Direct observation of the behaviors in question would help interpret and allow greater confidence in study results.
10 Baseline speeds were generally faster in Marion County than in Tucson. Average baseline speeds in 35 mph zones were 6.1 mph over the limit in Marion County, compared to 3.7 mph in Tucson; average speeds in 40 mph zones were 5.3 mph over the limit in Marion County, compared to 2 mph in Tucson. The causes and possible effects of the apparent differences in baseline speeds are unknown.