Review of Helmet and Helmet Law Effectiveness
Studies
By Jonathan Goldstein
I. Helmet Effectiveness Studies based on Variants of
Correlation Analysis Which Support Motorcycle Helmet
Effectiveness
Introduction
There are two types of studies in this category: (1) before-after
helmet law repeal studies, and (2) accident victim studies. The former
tests the difference between death rates, injury rates' location rates
of injuries, and severity rates of particular types of injuries (for all
accidents occurring in a particular geographic region) for a similar
period of time before and after helmet law repeal (Dare et al., 1979;
McSwain and Lummis, 1980). The latter test the difference between these
same rates for helmeted and non-helmeted accident victims (for a sample
of all accidents during a single time period) in a geographic locale
that does not have a mandatory helmet use law (Chang, 1981: Dare et al.,
1979; Heilman et al., 1982; Hurt et al. F 1981; Kraus et al., 1975; Luna
et al., 1981; Scott 1983). In each case statistically significant
differences are attributed to helmet use or non-use. Typical results
associated with this literature are death and injury rates two to three
times greater for non- helmeted accident victims and increases in
occurrence rates (death and injury) in repeal years that range from 19%
to 63%. Typical conclusions are that the non-use of a helmet are
responsible for these differences: helmet use is effective in the
reduction of death and injury.
The major limitation of these studies is the use of some variant of
correlation analysis as a statistical methodology. In general'
correlation analysis cannot be used to establish a causal mechanism
between any two variables (i.e. helmets and injury). This limitation of correlation analysis is universally
recognized and is not disputed. The fatal flaw in correlation
analysis is its inability to control for all
relevant factors that could be responsible for movements or
changes in a targeted (dependent variable) variable such as death or
injury. Failure to control for all such factors implies that a third
(uncontrolled for) factor could be responsible for the movements in one
or both of the correlated variables. Thus' the effects of a third factor
could erroneously be assigned to one of the two correlated variables
implying that the "causal" effect of that variable is either over or
understated.
In accident studies, the uncontrolled factor is the difference in
risk aversion between helmeted and non- helmeted riders. non-helmeted
riders are inherently more risky implying that on average they are more
likely to have higher speeds, crash speeds and alcohol consumption (see
Goldstein 1985) for statistical evidence). Given that speed and alcohol
are major determinants of death and injury in motorcycle accidents, the
failure of correlation analysis to control for these differences implies
that the death and injury inducing effect of higher speed and alcohol
levels will be inappropriately assigned to the non- use of a helmet.
Thus, helmet effectiveness is systematically
overstated by these types of studies.
The before-after designs fail to control for time as a third factor.
Dramatic time trends towards (1) lower median age and experience of
motorcycle owners , ( 2) higher average annual miles traveled, (3)
higher displacement machines, and (4) less strict enforcement of
federally mandated speed limits are not controlled for. Given that most
before-after studies are done during a period when these injury and
death inducing trends are dramatically increasing, these studies
erroneously assign these time related increases in death and injury to
the repeal of helmet laws which occur concurrently with these other
trends. Thus, before-after studies systematically
overstate the effectiveness of mandatory helmet use laws.
The most commonly used statistical methodology that can potentially
avoid the methodological flaws inherent in correlation analysis is
regression analysis. Unfortunately regression analysis is not a panacea
for the problems of correlation analysis. Regression equations must be
carefully specified in order to avoid specification bias which could
lead to the systematic over or understatement of helmet effectiveness.
Regression studies are analyzed below in Section
III.
In the reviews that follow references will be made to the concepts
developed in this introduction.
1. Hurt, H. H., Jr., Ouellet, J. V., and Thom, D. R. (1981, January)
Motorcycle Accident Cause Factors and Identification of Countermeasures,
Volume I: Technical Report, Volume II: Appendix/ Supplemental Data,
Contract DOT HS-5-01160. NHTSA, Washington, DC.
The "Hurt Study", a two-volume' 829 page report for the National
Highway Traffic Safety Administration (NHTSA) based on a $501,814 grant,
is the most widely referenced study on motorcycle accidents and
motorcycle safety issues. This accident victim study is based on data
from the on-scene, in-depth investigation of 900 motorcycle accidents in
the Los Angeles area for which 1,045 pieces of information (variables)
where collected for each accident. The most important contribution of
this study is its data collection. This study
provides the best and most comprehensive set of data on motorcycle
accidents available in the world. The quality and detail of
the data, compared to typical accident reports, was enhanced through the
use of an on-scene multi-disciplinary research team that collected more
complete and accurate accident reconstruction, engineering, medical,
environmental, protection system, and operator and passenger
information.
The main weakness of the study is its
statistical methodology. Comparisons of statistical
frequencies for helmeted and non helmeted riders and cross tabulations,
both forms of correlation analysis (criticized in above introduction),
are the statistical techniques used to establish "causal" relations. The
Hurt study's findings on protective equipment are that "the only
significant protective equipment is the qualified safety helmet, and it
is capable of a spectacular reduction of head injury frequency and
severity." In particular, helmeted riders (1) show significantly lower
injury frequency in all types of lesions -- helmeted riders comprised
39.4% of the sample and experienced 22.8% of all head and neck injuries;
(2) show significantly lower injury frequency at all levels of injury
severity; (3) are more likely to have no head or neck injuries; and (4)
are less likely to experience severe F critical and clearly fatal head
and neck injuries. While the Hurt study recognizes the theoretical
possibility for helmets to cause neck injuries (p. 292), the findings
indicate that "there is no world- shaking advantage or disadvantage of
motorcycle helmet use in relation to neck injury."
The study also recognizes (p. 281) that extreme accident conditions
present a formidable problem for head protection by helmets. Thus,
practical limitations exist. Other main findings of the study are that
the main causes of accidents and injury are: (1) lack of caution and
awareness of involved automobile drivers; (2) lack of motorcycle
conspicuity; (3) lack of operator skill; and (4) crash speed. The best
countermeasures proposed are: (1) motorcycle training; (2) better
licensing procedures; (3) use of safety helmets; (4) use of eye
protection; (5) use of headlamps during daytime; and (5) use of bright
upper torso garments.
The statistical methodology employed in this study systematically
overstates the effectiveness of helmets (see introduction). Thus, this
study does not provide any scientifically valid evidence in favor of
motorcycle helmet effectiveness. An alternative
study (Goldstein 1986, 1988) that uses the Hurt data and controls for
the uncontrolled factors in the Hurt study finds dramatically different
results concerning helmet effectiveness.
2. McSwain, N.E., and Lummis, M. (1980) "Impact of Repeal of
Motorcycle Helmet Law." Surgery, Gynecology, and Obstetrics 151: 215-24.
This study was supported by the Federal Department of Transportation:
Contract No. DOT-HS- 7-01563.
This paper conducts both a before-after study for the State of Kansas
using data from 1975 and 1976 and an accident victim study Limited to
three primary population centers in the state. The study finds: (1) a
19.4% increase in the crude accident rate (accidents/ 1,000
registrations) from 12.8 to 15.3; (2) a 63.3% increase in the crude
fatality rate (fatalities per 1,000 accidents) from 15.0 to 24.5; (3) a
95% increase in fatalities per 100,000 registrations from 19.2 to 37.5;
(4) a 333% greater fatality per 1,000 occupants for non helmeted riders;
and (5) a 51% increase in head injuries per 1,000 accidents after helmet
law repeal. The accident victim study found: (1) a 25% greater injuries
per 1,000 accident victims; (2) a 35% higher general body injury rate;
(3) a 106% greater head injury rate; and (4) a 67% increase in head
injury severity for non helmeted riders. The study also finds that fewer
neck injuries are associated with helmet use. The study concludes that
significant differences in death and injury rates based on helmet use
imply that mandatory helmet legislation significantly reduces death and
disability.
The statistical methodology is based on the comparison of
before-after and helmeted-non helmeted rates, a form of correlation
analysis. The accident victim study is subject to the above discussed
criticism. In addition, the inability to explain why non helmeted riders
have 35% more general body injuries highlights that the more risky
behavior of this group of riders has not been controlled for by the
simple correlation analysis. The before-after comparisons fail to
control for the time trends discussed above and the dramatic change in
economic conditions from the 1975 recession to the 1976 recovery which
affect riding patterns. In particular, the inability to explain a 19.5%
increase in the accident rate highlights these limitations.
3. Dare, C. E., Owens, J.C., and Krane, S. (1979) "Effect of
Motorcycle Safety Helmet Use on Injury Location and Severity:
Before-and-After Helmet Law Repeal in Colorado".
This paper conducts both a before-after helmet law repeal study
(1976-1977) and an accident victim study to determine the effect of
helmet usage on the location, number , and severity of motorcycle
injuries. The before- after study finds: (1) a 48% increase in total
accidents; and (2) a 140% increase in the number of fatalities. The
accident victim study finds: (1) an increase in the number of injuries
at all levels of injury severities except the most minor types of
injury; (2) a fatality rate 2.2 times greater; (3) a critical injury
rate 3.3 times greater; and (4) a head injury rate that is 2.6 times
greater for non helmeted riders. In addition the study shows that the
rate of the most severe neck injury decreased
slightly for non helmeted riders but, non helmeted riders had a neck
injury rate (for all neck injuries) 1.3 times greater than helmeted
riders. It is also found that non helmeted riders are over
represented in accidents.
The statistical methodology employed is the same as other studies in
this category. Failure to explain the 48% increase in total accidents
suggests that all relevant trends in the post-repeal period have not
been controlled for and failure to explain the over representation of
non helmeted riders in accidents implies that the inherently more risky
behavior of this group of riders has not been considered. Thus
overstated estimates of helmet effectiveness result.
4. Kraus, J. F., Riggins, R. S., and Franti, C. E. (1975). "Some
Epidemiologic Features of Motorcycle Collision Injuries." American
Journal of Epidemiology, 102:99-109.
This accident victim study (Sacramento County, California)
establishes the major factors associated with motorcycle injuries and
the severity of such injuries. Findings include: (1) the severity of
injury is positively related to age; (2) the frequency of serious head
injuries was highest for non helmeted drivers -- 14.1% of helmeted
riders compared to 22.8% of non helmeted riders had serious head
injuries; and (3) the severity of injury is positively related to crash
speed -- serious injuries increased when crash speeds exceeded 48/km/hr.
This study is good for isolating potential factors (age, speed, helmet
use) which should be included in an (regression) analysis that controls
for all factors associated with motorcycle injuries. The study, because
of its correlation methodology , is not capable of isolating the
individual effect of helmets in reducing injuries. Thus, the author's
conclusion that mandatory helmet laws are effective for reducing
fatalities and injuries is not scientifically supported.
5. Heilman, D. R., Weisbuch, J. B., Blair, R. W., and Graf, L. L.
(19821. "Motorcycle-Related Trauma and Helmet Usage in North Dakota."
Annuals of Emergency Medicine, 11:659-664.
This accident victim study (North Dakota) uses comparative
frequencies and cross tabulations to find that helmetless riders: (1)
suffer head, neck and face injuries at a rate 2.3 greater; (2) sustain
3.19 times more fatalities and 2.83 more life threatening injuries; and
(3) have more injuries and more severe injuries than helmeted riders.
Quoting other studies from Maryland and Colorado, the study addresses
the costs to society: the average medical costs without doctor's fees of
a motorcycle accident is estimated to be $11,038 and 53.7% of those
costs are paid by the taxpayer. The study also
recognizes that non helmeted riders are over represented in accidents
and that 50% of crash victims didn't have a valid license to operate a
motorcycle. The Hurt study has similar
findings on over representation and the unlicensed driver
Problem. Policy recommendations include mandatory helmet use
and stricter licensing requirements and enforcement. On the basis of its
flawed statistical methodology, this study inappropriately concludes
that helmets are effective and inappropriately infers that the costs to
society could be reduced by the imposition of a helmet law.
6. Luna, G. K., Copass, M. K., Oreskovich, M. R., and Carrico, C. J.
(1981). "The Role of Helmets in Reducing Head Injuries from Motorcycle
Accidents: A Political or Medical Issue?" The Western Journal of
Medicine, 135:89-92.
This accident victim study (Seattle) finds: (1) the death rate for helmeted and non helmeted was not
significantly different; (2) helmeted and unhelmeted fatal
accident victims had statistically identical total severity scores; (3)
the chance of sustaining a severe head injury was significantly higher
for non helmeted riders -- 82% of such injuries were sustained by
unhelmeted riders; and (4) the incidence of long-term neurological
damage from severe head injury was three times greater for non helmeted
riders. While this study supports typical findings on head injury
severity for non helmeted riders, it does contradict the findings of
earlier studies on a three times -greater incidence of sustaining a
fatal injury for non helmeted riders. The statistical methodology of
this study -- frequency comparisons -is subject to the same criticisms
discussed above.
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© Copyright Jonathan P. Goldstein Ph.D.
1986. All Rights
Reserved.
