1. This study employs standard statistical techniques (regression analysis) to isolate the main determinants of death and injury severity resulting from motorcycle accidents.
2. The data analyzed in this study were provided by the National Highway Traffic Safety Administration, U.S. Department of Transportation and originally collected by Hurt et al. (1981), contract No. DOT HS-5-01160. These data are currently recognized as the most accurate and detailed available on motorcycle accidents (See pp. 11-12).
3. The effectiveness of helmets and other determinants of death and injury severity are estimated from a causal model. Three variants of one causal model are used to isolate the determinants of: (1) the probability of a fatality; (2) the severity of head injuries; and (3) the severity of neck injuries.
4. The use of a causal model distinguishes the research methodology of this study from previous studies. The advantage of this approach lies in the ability to estimate the separate effects of several simultaneous and interrelated causes of motorcycle fatalities and injury severities (pp. 2-4). Previous studies simply divide accident victims into a helmeted group and non-helmeted group. As a result all differences in fatality rates, injury rates and injury severities between groups are erroneously attributed to helmet use. These comparisons fail to consider other differences between helmet users and non-users which influence the probability of death and the severity of injuries. The most plausible hypothesis is that helmeted riders are more risk-averse and thus: (1) have lower pre-crash and thus crash speeds; and (2) are less likely to combine alcohol consumption and driving. Such behavior, rather than helmet use per se, may dramatically reduce the probability of fatality or the severity of an injury. Only a causal model that considers crash speed, helmet use, alcohol use and other pertinent variables can isolate the separate contribution of each determinant of the severity of injury or probability of death.
Causal Model (pp. 4-8)
1. The causal model considers three broad categories of the causes of death and injury severity. These include factors governed by the laws of physics, physiological factors. and human factors and operator characteristics.
2. The physical factors considered include: the kinetic energy (potential for bodily damage) transferred to the motorcycle operator by the impact, compressibility of the impacted object, helmet use, and possible engineering limitations of helmets (as affected by the impact speed that the helmet is subjected to in the crash).
3. The physiological factors considered include: operator's age, blood alcohol level, drug involvement, and permanent physiological impairment.
4. The human factors and operator characteristics considered include: rider on-road experience, whether the operator had taken the correct evasive action for the particular accident situation, driver training, and the operator's past accident and violation histories.
5. Numerous other determinants were also considered.
Results (pp. 13-18)
1. Helmets are shown to have no statistically significant effect on the probability of a fatality given that a motorcycle accident has occurred. This means that based on standard statistical tests we cannot reject the claim that helmets do not affect the probability that a rider will survive a motorcycle accident.
2. The major determinants of fatality are the rider's crash speed (kinetic energy) and blood alcohol level.
3. For the average rider involved in the average accident, it is found that the probability of death increases from 2.1% to 11.3% when the rider's blood alcohol level increases from 0.0 to 0.1 (from sober to legally intoxicated in most states).
4. In the same vein, an increase in the crash speed from 40 to 60 mph increases the probability of death from 7.1% to 36.3%
5. It is found that helmets have a statistically significant effect in reducing head injury severity. We can reject the hypothesis that helmets have no effect on head injuries in favor of the claim that they reduce head injuries.
6. It is shown that past a critical impact velocity to the helmet (approximately 13 mph), helmet use has a statistically significant effect which increases the severity of neck injuries. Thus we reject the claim that, helmets have no effect on neck injuries in favor of the claim that, past a critical impact speed, they exacerbate neck injuries.
7. As a result of (5) and (6), we establish that a tradeoff between head and neck injuries confronts a potential helmet user. Past a critical impact speed to the helmet (13 mph), which is likely to occur in real life accident situations helmet use reduces the severity of head injuries at the expense of increasing the severity of neck injuries.
8. Further statistical tests reveal the qualitative nature of this tradeoff. It is shown that an individual who wears a helmet and experiences an impact velocity to the head greater than 13 mph may avoid either severe or minor head injuries and incur either severe or minor neck injuries; all permutations of the tradeoff are equally likely to occur.
Policy Implications (pp. 18-20)
1. If a major concern of policy makers is the prevention of fatalities, helmet legislation may not be effective in achieving that objective.
2. If the overall cost to society of motorcycle accidents is the issue, then cost-benefit analyses that adequately consider the tradeoff between head and neck injuries must be conducted before the cost effectiveness of helmets can be determined.
3. Until the injury tradeoff issue is more carefully studied, it cannot be concluded that mandatory helmet use laws are an effective method to eradicate the slaughter and maiming, of individuals involved in motorcycle accidents.
4. A more effective policy approach would be two pronged, including both policies to prevent accidents and policies that effectively reduce the probability of death and the severity of injuries.
5. Policies to prevent accidents include: (1) the education of the general driving public; (2) the education of a younger and more inexperienced population of motorcyclists on the issues of accident avoidance and the proper use and control of high horsepower machines: (3) stricter enforcement of drunk driving laws; and (4) implementation of alcohol awareness programs.
6. Policies to reduce death and injury severity include: stricter enforcement of speed limits. the alcohol related policies suggested in (5) and mandatory driver training and education programs which emphasize the proper execution of evasive action.