Index
Documentation Page
Summary
Background
Methods
Results
Countermeasures
Tri-Level Comparison
Reference

Appendix

Comparison of UDA and Indiana Tri-Level Causal Analyses

In this section, the UDA causal analysis results are compared with the Indiana Tri-Level analysis results (Treat, et al, 1979). There are several factors to be considered in reviewing comparison results:

  • The focus of the Indiana Tri-Level study was identification of all factors related to crash occurrence. In contrast, the focus of the UDA study was identification of problem driving behaviors and identification of situational factors/characteristics associated with these behaviors. The more limited research objective of the UDA study was likely to result in an underreporting of environmental and vehicle factors as compared to the Tri-Level study or other more global studies of causation factors.

  • A significant portion of the vehicle related factors in the Tri-Level study were related to braking system deficiencies (30.8 percent). The specific deficiencies noted in that study (e.g., gross failures, side-to-side imbalances, premature lock-up, etc.) occur at much lower frequency levels in the more advanced braking systems installed in vehicles manufactured in the 1990s (most UDA study vehicles).

  • The UDA study did not utilize the "certain," "probable," and "possible" levels to describe causal assignments. UDA causal assignments were most directly comparable to the probable level assignments made by the on-site teams in the Tri-Level study.

A comparison of human, environment, and vehicle causal factors assigned in these two studies is provided in Figure 4. As was anticipated, there was a pronounced disparity in the assigned levels of environment and vehicle factors in the two studies. While the levels of disparity were primarily related to the more limited research objectives of the UDA study, improvements in vehicle system designs may have also contributed to the very low level of vehicle factors noted in the UDA study.

A comparison of the six most frequently assigned human-related causal factors in the two studies is provided in Figure 5. The UDA incidence rates are converted from the proportion of drivers contributing to the proportion of crashes used in the Tri-Level study. The upper portion of Figure 5 provides a comparison of the four causal groups that were among the six most frequently assigned causal factors in both studies. The mid portion of the figure provides a comparison of two causal factors that were part of the six most frequently assigned causal factors in the UDA study, but that did not appear in the six most frequently assigned causal factors in the Tri-Level study. Finally, the lower portion of the figure provides a comparison of two causal factors that were part of the six most frequently assigned causal factors in the Tri-Level study, but that did not appear in a similar distribution for the UDA study. Major findings may be summarized as follows:

Four Common Causal Factor Groups

  • The driver inattention category, as defined in the UDA study, was comprised of the driver inattention and driver distraction categories as defined in the Tri-Level study. This factor was assigned to 23.0 percent of the crashes in the UDA study and 20.3 percent of the crashes in the Tri-Level study.

  • The excessive speed category was assigned to 18.9 percent of the crashes in the UDA study and 14.7 percent of the crashes in the Tri-Level study.

  • The UDA perceptual error category (15.3 percent) was directly comparable to the Tri-Level improper lookout category (20.3 percent). Both category labels were somewhat arbitrary in nature. It is also interesting to note that both studies found an over-representation of older drivers in this category.

  • The UDA decision error category (10.1 percent) was directly comparable to Tri-Level false assumption category (11.8 percent).

  • In general, these four common factors demonstrated a remarkable degree of consistency over time. Specifically, these factors were assigned to 67.4 percent of the UDA crashes and 66.8 percent of the Tri-Level crashes.

UDA Alcohol Impairment and Incapacitation Factors

  • The alcohol impairment factor was assigned to 18.4 percent of the UDA crashes and only 6.1 percent of the Tri-Level crashes. As stated in the Tri-Level report, that study experienced a very high incidence rate of property damage only crashes. The report authors believed that this large property damage incidence rate accounted for the relatively low level of alcohol related crashes. On the other hand, the UDA study had an overrepresentation of serious injury crashes. Other studies of injury crashes (Terhune and Fell, 1981) show alcohol involvement at about 20 percent.

    1. The UDA incapacitation category (comprised of drivers who fell asleep or experienced a heart attack, seizure, or blackout) was assigned to 6.5 percent of the UDA crashes and was comparable to the Tri-Level critical non-performance category which was assigned to 1.4 percent of the Tri-Level crashes. The UDA rate is consistent with other causal analyses completed with NASS data. The relatively low rate reported in the Tri-Level study may again be related to the high incidence of property damage only crashes in that study.

Tri-Level Improper Evasive Action and Improper Maneuver Factors

  • The improper evasive action category was assigned to 10.3 percent of the Tri-Level crashes and 2.1 percent of the UDA crashes.

  • The improper maneuver category was assigned to 7.1 percent of the Tri-Level crashes and 3.4 percent of the UDA crashes.

  • The disparity level in the assignment frequencies for these categories appeared to be associated with the classification scheme used to designate alcohol-related crashes in the UDA study. In this effort, these behaviors were assumed to be part of the alcohol designation. Specifically, the only additional factors that were routinely recorded in alcohol-related crashes in the UDA study were excessive vehicle speed and traffic control device violations. A clinical review of a sample of UDA alcohol-related crashes indicated that if these factors were added to the alcohol designation, the UDA incidence rate for improper evasive action would increase by a factor of two to three times and the incidence rate for improper maneuver would nearly double in size.