This article was downloaded by: [University of Auckland Library] On: 19 October 2014, At: 17:33 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Traffic Injury Prevention Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gcpi20
Predictors of Nonstandard Helmet Use Among San Francisco Bay–Area Motorcyclists a
b
Casey K. Tsui , Thomas M. Rice & Swati Pande a
b
Human Impact Partners , Oakland , California
b
Safe Transportation Research & Education Center, University of California–Berkeley , Berkeley , California Accepted author version posted online: 05 Aug 2013.Published online: 17 Dec 2013.
Click for updates To cite this article: Casey K. Tsui , Thomas M. Rice & Swati Pande (2014) Predictors of Nonstandard Helmet Use Among San Francisco Bay–Area Motorcyclists, Traffic Injury Prevention, 15:2, 151-155, DOI: 10.1080/15389588.2013.796041 To link to this article: http://dx.doi.org/10.1080/15389588.2013.796041
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Traffic Injury Prevention (2014) 15, 151–155 C Taylor & Francis Group, LLC Copyright ISSN: 1538-9588 print / 1538-957X online DOI: 10.1080/15389588.2013.796041
Predictors of Nonstandard Helmet Use Among San Francisco Bay–Area Motorcyclists CASEY K. TSUI1, THOMAS M. RICE2, and SWATI PANDE2 1 2
Human Impact Partners, Oakland, California Safe Transportation Research & Education Center, University of California–Berkeley, Berkeley, California
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Received 6 September 2012, Accepted 11 April 2013
Objective: The use of helmets that do not comply with safety standards is common in California. The objective of this study was to describe the use of these nonstandard helmets among San Francisco Bay–area (SFBA) motorcyclists and to identify personal and motorcycle characteristics that are associated with the use of nonstandard helmets. Methods: A survey of 860 SFBA motorcyclists was conducted. Log-binomial regression was used to estimate risk ratios to compare probabilities of nonstandard helmet use. Results: Fifteen percent of motorcyclists reported wearing a nonstandard helmet sometimes or often. BMW riders had the lowest use of nonstandard helmet (5%) and Harley-Davidson riders had the highest use (51%). Among non-Harley-Davidsons, riders of cruiser-style motorcycles were 3.1 times as likely to wear a nonstandard helmet as riders of motorcycles of other styles. African American riders were more than twice as likely to use nonstandard helmets compared to riders with other self-reported race. Discussion: Behavioral countermeasures are needed to improve motorcycle helmet choice in California. This study identified riders of Harley-Davidsons and riders of cruiser-style motorcycles of other brands as potential targets of interventions. Keywords: motorcycles, California, survey, helmets, accidents, traffic
Introduction In the United States between 1998 and 2008, the number of motorcyclists fatally injured in traffic collisions rose from 2,294 to 5,290, a 131 percent increase. During the same period, the number of fatalities among passenger vehicle occupants fell from 31,899 to 25,351, a 21 percent decrease (National Highway Traffic Safety Administration 2009). In addition, nonfatal injury collisions are on the rise among motorcyclists. In California, the number of non-fatally injured motorcyclists increased from 9,488 in 2004 to 11,764 in 2008, a 24 percent increase. Much of the increase in motorcyclist injuries and fatalities has resulted from sharp increases in motorcycle ridership. Motorcycle registrations in California increased 99 percent between 1997 and 2008 (personal communication, Scott Master, California Department of Motor Vehicles, December 2009). California has had a universal helmet law since 1991 (California Vehicle Code §27803 1991) mandating the use of helmets that comply with Federal Motor Vehicle Safety Standard (FMVSS) 218(U.S. Department of Transportation), referred to as Department of Transportation (DOT)-compliant helmets. Enforcement has proven difficult in California because Address correspondence to Thomas M. Rice, Safe Transportation Research & Education Center, University of California–Berkeley, 2614 Dwight Way #7374, Berkeley, CA 94720-7374. E-mail: [email protected]
of court decisions that law enforcement officers cannot determine whether a helmet meets the FMVSS 218 standard and that only the manufacturer or a qualified laboratory can determine compliance. Officers must rely solely on the presence of a DOT label on the helmet. The result of these decisions is that any helmet, regardless of construction, that has a DOT label will rarely be cited by California law enforcement officers, allowing motorcyclists to wear any style helmet with a DOT label supplied with the helmet or purchased separately and applied by the rider. It is not uncommon in California to observe riders wearing helmets with little or no energy-absorbing polystyrene or helmets designed for baseball, football, or similar activities. Many of these helmets bear a DOT label even though they clearly would not meet the FMVSS standard if they were tested. These helmets are often referred to as bogus helmets; we refer to them as nonstandard helmets throughout this article. Motorcycle helmets have long been recognized as an effectiveness countermeasure to prevent head and brain injury among collision-involved motorcycle riders (Houston and Richardson 2007; Mertz and Weiss 2008; Norvell and Cummings 2002; Rowland et al. 1996; Vaca 2006). Helmet effectiveness does appear to vary across helmet styles, though, with some providing little or no protection (Peek-Asa et al. 1999). Because properly designed helmets are so effective in preventing severe brain injury (Liu et al. 2009), efforts to improve helmet choice among motorcyclists are likely to lead
152 to overall safety improvements. Programs to improve helmet choice among motorcyclists in states with mandated helmet use may be more cost-effective if they can specifically target motorcyclists with high rates of nonstandard helmet use. This study describes patterns of nonstandard helmet use among San Francisco Bay–area (SFBA) motorcyclists and estimates associations between rider and motorcycle characteristics with the use of nonstandard helmets.
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Methods A survey of motorcyclists was conducted using computerassisted personal interviewing field interviews, computerassisted personal interviewing phone interviews, webadministered surveys, and mail-back paper surveys between August 23, 2008, and October 6, 2008. Eligible participants were aged 18 years or older, regularly traveled by motorcycle, lived in the SFBA, and spoke English. This study was approved by the institutional review board at the University of California Berkeley. Motorcyclists were sampled using 3 methods: interviewing recreational riders at a primary recreational riding area in the SFBA, distributing 2096 leaflets on motorcycles parked in public commuter areas and residential neighborhood streets in 6 SFBA cities (San Francisco, Stanford, San Jose, Oakland, Berkeley, and Richmond), and recruiting on Internet bulletin boards and forums specific to the SFBA. With the exception of field-interviewed riders, participants were allowed to choose between web- or phone-administered interviews, and those who completed the interview received $20. For neighborhood leafleting, census tract data were used to identify 8 tracts from each city to provide variation in housing density and mean income. The survey elicited information on rider demographics, motorcycle make and model, types and frequency of helmets worn, the use of formal and informal training resources, and type and amount of riding. Helmets of different types were described to motorcyclists as follows: full-face, open-face, modular (convertible between full- and open-face), half-shell with padding, half-shell without padding, and beanie/skull cap. For each helmet type, respondents stated whether they wore that type of helmet often, sometimes, or never. Riders who stated that they wore half-shell without padding or beanie/skull cap–type helmets sometimes or often were categorized as nonstandard helmet users. Motorcycles were classified as cruisers or noncruisers. Cruiser-style motorcycles were characterized by most (but not necessarily all) of the following characteristics: low seat height, low center of gravity, forward foot peg position, upright seating position, V-twin engine, and vintage styling. The proportion of motorcyclists who wore nonstandard helmets and binomial exact confidence intervals (CIs) were calculated. Log-binomial regression was used to estimate risk ratios (RRs), which are ratios of the probability of nonstandard helmet use. Log-binomial regression has been advocated as an alternative to logistic regression when RRs (or prevalence ratios) can be estimated directly, as is the case with crosssectional data or cohort data (Cummings 2004; Deddens and
Tsui et al. Petersen 2004; Skov et al. 1998; Wacholder 1986). In some study designs, the estimation of odds ratios, usually with logistic regression, is useful because risk or prevalence ratios cannot be estimated and are instead approximated with odds ratios. Odds ratios will, under conditions of a rare outcome, generally approximate RRs. When the outcome is common, as in our data, odds ratios can be much further from the null than the desired risk or prevalence ratio. Covariates in our models included contact method, age, race, Hispanic ethnicity, brand of primary motorcycle, motorcycle style, completion of formal rider training, years of riding experience, and kilometers ridden per year. Respondent sex was not elicited in the questionnaires. A sensitivity analysis of the possible effect of omitting sex from the models was performed by fitting models with and without a sex variable derived from first name (when unambiguously determinable). It was demonstrated that sex was not a meaningful predictor of nonstandard helmet use nor was it a confounder of the other estimated RRs. All statistical analyses were conducted using the Stata statistical package (StataCorp 2012).
Results A total of 860 motorcyclists completed the survey (74 phone/mail, 517 web, and 269 field). Median age was 38 years old and 26 percent of riders were non-white (Table 1). Mean years of riding experience was 14 (median = 9), and 23 percent reported owning more than one motorcycle. Sixty-two percent of riders reported having completed a training course. Of those who did not, 31 percent said that they also received no informal training. Overall, 131 riders reported wearing a nonstandard helmet sometimes or often. The use of nonstandard helmets varied greatly by motorcycle brand and by whether the motorcycle was a cruiser-style motorcycle (Table 2). Cruiser riders, overall, were more likely to report wearing a nonstandard helmet (35.8%) compared to noncruiser riders (10.4%). The use of nonstandard helmets varied greatly by the brand of the riders’ primary motorcycle, ranging from 4.7 percent for BMW riders to 51.1 percent for Harley-Davidson riders. For riders of most brands, those who rode cruisers were more likely to use a nonstandard helmet. For example, 37.5 percent of Yamaha cruiser riders wore nonstandard helmets, compared to 9.9 percent of Yamaha noncruiser riders. This trend was observed for all brands except Harley-Davidson for which nonstandard helmet use was high regardless of style: 49.2 percent of HarleyDavidson cruiser riders and 57.1 percent of Harley-Davidson noncruiser riders. Adjusted RRs and CIs from the multivariate regression are shown in Table 3. Race, motorcycle brand, and years of riding experience were significantly associated with nonstandard helmet use. Riders with self-reported black race had an estimated RR of 2.15 (95% CI: 1.01, 4.56). Using BMW brand (with the lowest prevalence of nonstandard helmet use) as the referent, Harley-Davidson, Honda, and Kawasaki riders had the highest RRs: 11.04 (95% CI: 4.20, 29.06), 3.62 (95% CI: 1.28, 10.20), and 5.48 (95% CI: 1.91, 15.73), respectively. All categories of years of riding experience greater than 3 years
Helmet Use Predictors
153
Table 1. Motorcyclist characteristics, San Francisco Bay Area Motorcycle Survey, 2008
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Characteristic Age (years) 18–24 25–34 35–44 45 or older Unknown Race/ethnicity White Black Asian Other Hispanic heritage No Yes Cruiser motorcycle No Yes Years of riding experience Less than 3 3–10 11–25 25 or more Kilometers ridden per year Less than 3219 3219–8045 8047–14,482 14,484 or more Completed formal training course No Yes Total
No. (%) 62 (7) 265 (31) 244 (28) 257 (30) 32 (4) 604 (70) 14 (2) 76 (9) 166 (19) 766 (89) 94 (11) 701 (82) 159 (18) 233 (27) 232 (27) 219 (26) 176 (20) 195 (23) 224 (26) 226 (26) 215 (25) 322 (37) 538 (63) 860 (100)
were associated with higher prevalence of nonstandard helmet use. For example, riders with 3 to 10 years of experience had an estimated RR of 2.55 (95% CI: 1.32, 4.91). Rider age (P = .51), Hispanic ethnicity (P = .79), and kilometers ridden per year (P = .62) were not significantly associated with nonstandard helmet use, and having taken a formal training course had an association with nonstandard helmet use of marginal significance (P = .09). A strong interaction between motorcycle brand and cruiser style was observed (P < .001; Table 4). Among non-HarleyDavidson riders, cruiser riders were 3.1 times as likely as noncruiser riders to use a nonstandard helmet, whereas among Harley-Davidson riders, the likelihood of nonstandard helmet use did not differ significantly (RR = 4.71/5.46 = 0.86; 95% CI: 0.51–1.46; P = .585).
Discussion This study was conducted to obtain a snapshot of the helmet use patterns of motorcyclists in the SFBA—a major, heavily trafficked metropolitan region of California—and to identify rider characteristics associated with nonstandard, or bogus, helmet use. The SFBA is a motorcycling-intensive region of California; of the 9 counties that comprise the SFBA, 7 have per capita motorcycle registration rates higher than the California statewide rate and 2 have rates equal to the statewide
rate. (personal communication, Scott Masten, California Department of Motor Vehicles, December 2009; California Department of Finance 2009) Overall, 15 percent of surveyed motorcyclists reported wearing a nonstandard helmet sometimes or often. However, the prevalence of nonstandard helmet use differed widely across groups. Black riders and those with greater riding experience were more likely to wear nonstandard helmets. More important, motorcyclists who rode cruiser-style motorcycles were more than twice as likely to report nonstandard helmet use as riders of other motorcycles, after adjusting for brand. We observed that our cruiser RR estimate was confounded by motorcycle brand. This confounding appears to be driven by the Harley-Davidson brand. Harley-Davidson motorcycles were much more likely to be cruiser-style motorcycles and Harley-Davidson riders were much more likely to report wearing a nonstandard helmet. The cruiser RR unadjusted for brand was 3.3 and dropped to 1.9 after adjustment for brand. Reported use of nonstandard helmets varied greatly by brand. BMW riders had the lowest use of nonstandard helmets (5%) and Harley-Davidson riders had the highest use (51%). The magnitude of the estimated brand RRs is notable. Harley-Davidson riders were 11 times as likely to use nonstandard helmets as BMW riders. Three Japanese brands (Honda, Kawasaki, and Yamaha) had large and statistically significant associations with nonstandard helmet use. One common Japanese brand, Suzuki, was not significantly associated with nonstandard helmet use. The association between cruisers and nonstandard helmet use can be seen in the case of Honda and Yamaha, the Japanese manufacturers with, by far, the highest proportion of cruiser motorcycles in our sample—22 and 23 percent, respectively. Table 2. Number of motorcyclists and percentage reporting nonstandard helmet use by motorcycle make, San Francisco Bay Area Motorcycle Survey, 2008 Make BMW
Cruiser style Number % Nonstandard helmet
No Yes Total Harley-Davidson No Yes Total Honda No Yes Total Kawasaki No Yes Total Suzuki No Yes Total Yamaha No Yes Total Other No Yes Total Total No Yes Total
104 2 106 21 65 86 163 45 208 77 7 84 118 9 127 81 24 105 137 7 144 701 159 860
4.8 0 4.7 57.1 49.2 51.1 10.4 22.2 13.0 15.6 42.9 17.9 6.8 0 6.3 9.9 37.5 16.2 8.0 42.9 9.7 10.4 35.8 15.1
95% CI 1.6–10.9 — 1.5–10.7 34.0–78.2 36.6–61.9 40.1–62.1 6.2–16.2 11.2–37.1 8.7–18.3 8.3–25.6 9.9–81.6 10.3–27.7 3.0–12.9 — 2.8–12.0 4.4–18.5 18.8–59.4 9.7–24.7 4.1–13.9 9.9–81.6 5.4–15.8 8.3–12.9 28.4–43.8 12.8–17.7
154
Tsui et al.
Table 3. Risk ratios for nonstandard helmet use, San Francisco Bay Area Motorcycle Survey, 2008
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Characteristic
No. (%)
RRa
95% CI
P
Age (years) 18–24 62(7) Ref. — — 25–34 265(31) 0.79 0.28–2.24 .653 35–44 244(28) 1.10 0.38–3.22 .864 45 or older 257(30) 0.90 0.29–2.77 .849 Unknown 32(4) — — — Race/ethnicity White 604(70) Ref. — Black 14(2) 2.15 1.01–4.56 .047 Asian 76(9) 1.07 0.54–2.13 .845 Other 166(19) 1.52 0.92–2.51 .099 Hispanic heritage No 766(89) Ref. — — Yes 94(11) 0.93 0.54–1.60 .787 Motorcycle manufacturer BMW 106(12) Ref. — — Harley-Davidson 89(10) 11.04 4.20–29.06
Traffic Injury Prevention Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gcpi20
Predictors of Nonstandard Helmet Use Among San Francisco Bay–Area Motorcyclists a
b
Casey K. Tsui , Thomas M. Rice & Swati Pande a
b
Human Impact Partners , Oakland , California
b
Safe Transportation Research & Education Center, University of California–Berkeley , Berkeley , California Accepted author version posted online: 05 Aug 2013.Published online: 17 Dec 2013.
Click for updates To cite this article: Casey K. Tsui , Thomas M. Rice & Swati Pande (2014) Predictors of Nonstandard Helmet Use Among San Francisco Bay–Area Motorcyclists, Traffic Injury Prevention, 15:2, 151-155, DOI: 10.1080/15389588.2013.796041 To link to this article: http://dx.doi.org/10.1080/15389588.2013.796041
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Traffic Injury Prevention (2014) 15, 151–155 C Taylor & Francis Group, LLC Copyright ISSN: 1538-9588 print / 1538-957X online DOI: 10.1080/15389588.2013.796041
Predictors of Nonstandard Helmet Use Among San Francisco Bay–Area Motorcyclists CASEY K. TSUI1, THOMAS M. RICE2, and SWATI PANDE2 1 2
Human Impact Partners, Oakland, California Safe Transportation Research & Education Center, University of California–Berkeley, Berkeley, California
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Received 6 September 2012, Accepted 11 April 2013
Objective: The use of helmets that do not comply with safety standards is common in California. The objective of this study was to describe the use of these nonstandard helmets among San Francisco Bay–area (SFBA) motorcyclists and to identify personal and motorcycle characteristics that are associated with the use of nonstandard helmets. Methods: A survey of 860 SFBA motorcyclists was conducted. Log-binomial regression was used to estimate risk ratios to compare probabilities of nonstandard helmet use. Results: Fifteen percent of motorcyclists reported wearing a nonstandard helmet sometimes or often. BMW riders had the lowest use of nonstandard helmet (5%) and Harley-Davidson riders had the highest use (51%). Among non-Harley-Davidsons, riders of cruiser-style motorcycles were 3.1 times as likely to wear a nonstandard helmet as riders of motorcycles of other styles. African American riders were more than twice as likely to use nonstandard helmets compared to riders with other self-reported race. Discussion: Behavioral countermeasures are needed to improve motorcycle helmet choice in California. This study identified riders of Harley-Davidsons and riders of cruiser-style motorcycles of other brands as potential targets of interventions. Keywords: motorcycles, California, survey, helmets, accidents, traffic
Introduction In the United States between 1998 and 2008, the number of motorcyclists fatally injured in traffic collisions rose from 2,294 to 5,290, a 131 percent increase. During the same period, the number of fatalities among passenger vehicle occupants fell from 31,899 to 25,351, a 21 percent decrease (National Highway Traffic Safety Administration 2009). In addition, nonfatal injury collisions are on the rise among motorcyclists. In California, the number of non-fatally injured motorcyclists increased from 9,488 in 2004 to 11,764 in 2008, a 24 percent increase. Much of the increase in motorcyclist injuries and fatalities has resulted from sharp increases in motorcycle ridership. Motorcycle registrations in California increased 99 percent between 1997 and 2008 (personal communication, Scott Master, California Department of Motor Vehicles, December 2009). California has had a universal helmet law since 1991 (California Vehicle Code §27803 1991) mandating the use of helmets that comply with Federal Motor Vehicle Safety Standard (FMVSS) 218(U.S. Department of Transportation), referred to as Department of Transportation (DOT)-compliant helmets. Enforcement has proven difficult in California because Address correspondence to Thomas M. Rice, Safe Transportation Research & Education Center, University of California–Berkeley, 2614 Dwight Way #7374, Berkeley, CA 94720-7374. E-mail: [email protected]
of court decisions that law enforcement officers cannot determine whether a helmet meets the FMVSS 218 standard and that only the manufacturer or a qualified laboratory can determine compliance. Officers must rely solely on the presence of a DOT label on the helmet. The result of these decisions is that any helmet, regardless of construction, that has a DOT label will rarely be cited by California law enforcement officers, allowing motorcyclists to wear any style helmet with a DOT label supplied with the helmet or purchased separately and applied by the rider. It is not uncommon in California to observe riders wearing helmets with little or no energy-absorbing polystyrene or helmets designed for baseball, football, or similar activities. Many of these helmets bear a DOT label even though they clearly would not meet the FMVSS standard if they were tested. These helmets are often referred to as bogus helmets; we refer to them as nonstandard helmets throughout this article. Motorcycle helmets have long been recognized as an effectiveness countermeasure to prevent head and brain injury among collision-involved motorcycle riders (Houston and Richardson 2007; Mertz and Weiss 2008; Norvell and Cummings 2002; Rowland et al. 1996; Vaca 2006). Helmet effectiveness does appear to vary across helmet styles, though, with some providing little or no protection (Peek-Asa et al. 1999). Because properly designed helmets are so effective in preventing severe brain injury (Liu et al. 2009), efforts to improve helmet choice among motorcyclists are likely to lead
152 to overall safety improvements. Programs to improve helmet choice among motorcyclists in states with mandated helmet use may be more cost-effective if they can specifically target motorcyclists with high rates of nonstandard helmet use. This study describes patterns of nonstandard helmet use among San Francisco Bay–area (SFBA) motorcyclists and estimates associations between rider and motorcycle characteristics with the use of nonstandard helmets.
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Methods A survey of motorcyclists was conducted using computerassisted personal interviewing field interviews, computerassisted personal interviewing phone interviews, webadministered surveys, and mail-back paper surveys between August 23, 2008, and October 6, 2008. Eligible participants were aged 18 years or older, regularly traveled by motorcycle, lived in the SFBA, and spoke English. This study was approved by the institutional review board at the University of California Berkeley. Motorcyclists were sampled using 3 methods: interviewing recreational riders at a primary recreational riding area in the SFBA, distributing 2096 leaflets on motorcycles parked in public commuter areas and residential neighborhood streets in 6 SFBA cities (San Francisco, Stanford, San Jose, Oakland, Berkeley, and Richmond), and recruiting on Internet bulletin boards and forums specific to the SFBA. With the exception of field-interviewed riders, participants were allowed to choose between web- or phone-administered interviews, and those who completed the interview received $20. For neighborhood leafleting, census tract data were used to identify 8 tracts from each city to provide variation in housing density and mean income. The survey elicited information on rider demographics, motorcycle make and model, types and frequency of helmets worn, the use of formal and informal training resources, and type and amount of riding. Helmets of different types were described to motorcyclists as follows: full-face, open-face, modular (convertible between full- and open-face), half-shell with padding, half-shell without padding, and beanie/skull cap. For each helmet type, respondents stated whether they wore that type of helmet often, sometimes, or never. Riders who stated that they wore half-shell without padding or beanie/skull cap–type helmets sometimes or often were categorized as nonstandard helmet users. Motorcycles were classified as cruisers or noncruisers. Cruiser-style motorcycles were characterized by most (but not necessarily all) of the following characteristics: low seat height, low center of gravity, forward foot peg position, upright seating position, V-twin engine, and vintage styling. The proportion of motorcyclists who wore nonstandard helmets and binomial exact confidence intervals (CIs) were calculated. Log-binomial regression was used to estimate risk ratios (RRs), which are ratios of the probability of nonstandard helmet use. Log-binomial regression has been advocated as an alternative to logistic regression when RRs (or prevalence ratios) can be estimated directly, as is the case with crosssectional data or cohort data (Cummings 2004; Deddens and
Tsui et al. Petersen 2004; Skov et al. 1998; Wacholder 1986). In some study designs, the estimation of odds ratios, usually with logistic regression, is useful because risk or prevalence ratios cannot be estimated and are instead approximated with odds ratios. Odds ratios will, under conditions of a rare outcome, generally approximate RRs. When the outcome is common, as in our data, odds ratios can be much further from the null than the desired risk or prevalence ratio. Covariates in our models included contact method, age, race, Hispanic ethnicity, brand of primary motorcycle, motorcycle style, completion of formal rider training, years of riding experience, and kilometers ridden per year. Respondent sex was not elicited in the questionnaires. A sensitivity analysis of the possible effect of omitting sex from the models was performed by fitting models with and without a sex variable derived from first name (when unambiguously determinable). It was demonstrated that sex was not a meaningful predictor of nonstandard helmet use nor was it a confounder of the other estimated RRs. All statistical analyses were conducted using the Stata statistical package (StataCorp 2012).
Results A total of 860 motorcyclists completed the survey (74 phone/mail, 517 web, and 269 field). Median age was 38 years old and 26 percent of riders were non-white (Table 1). Mean years of riding experience was 14 (median = 9), and 23 percent reported owning more than one motorcycle. Sixty-two percent of riders reported having completed a training course. Of those who did not, 31 percent said that they also received no informal training. Overall, 131 riders reported wearing a nonstandard helmet sometimes or often. The use of nonstandard helmets varied greatly by motorcycle brand and by whether the motorcycle was a cruiser-style motorcycle (Table 2). Cruiser riders, overall, were more likely to report wearing a nonstandard helmet (35.8%) compared to noncruiser riders (10.4%). The use of nonstandard helmets varied greatly by the brand of the riders’ primary motorcycle, ranging from 4.7 percent for BMW riders to 51.1 percent for Harley-Davidson riders. For riders of most brands, those who rode cruisers were more likely to use a nonstandard helmet. For example, 37.5 percent of Yamaha cruiser riders wore nonstandard helmets, compared to 9.9 percent of Yamaha noncruiser riders. This trend was observed for all brands except Harley-Davidson for which nonstandard helmet use was high regardless of style: 49.2 percent of HarleyDavidson cruiser riders and 57.1 percent of Harley-Davidson noncruiser riders. Adjusted RRs and CIs from the multivariate regression are shown in Table 3. Race, motorcycle brand, and years of riding experience were significantly associated with nonstandard helmet use. Riders with self-reported black race had an estimated RR of 2.15 (95% CI: 1.01, 4.56). Using BMW brand (with the lowest prevalence of nonstandard helmet use) as the referent, Harley-Davidson, Honda, and Kawasaki riders had the highest RRs: 11.04 (95% CI: 4.20, 29.06), 3.62 (95% CI: 1.28, 10.20), and 5.48 (95% CI: 1.91, 15.73), respectively. All categories of years of riding experience greater than 3 years
Helmet Use Predictors
153
Table 1. Motorcyclist characteristics, San Francisco Bay Area Motorcycle Survey, 2008
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Characteristic Age (years) 18–24 25–34 35–44 45 or older Unknown Race/ethnicity White Black Asian Other Hispanic heritage No Yes Cruiser motorcycle No Yes Years of riding experience Less than 3 3–10 11–25 25 or more Kilometers ridden per year Less than 3219 3219–8045 8047–14,482 14,484 or more Completed formal training course No Yes Total
No. (%) 62 (7) 265 (31) 244 (28) 257 (30) 32 (4) 604 (70) 14 (2) 76 (9) 166 (19) 766 (89) 94 (11) 701 (82) 159 (18) 233 (27) 232 (27) 219 (26) 176 (20) 195 (23) 224 (26) 226 (26) 215 (25) 322 (37) 538 (63) 860 (100)
were associated with higher prevalence of nonstandard helmet use. For example, riders with 3 to 10 years of experience had an estimated RR of 2.55 (95% CI: 1.32, 4.91). Rider age (P = .51), Hispanic ethnicity (P = .79), and kilometers ridden per year (P = .62) were not significantly associated with nonstandard helmet use, and having taken a formal training course had an association with nonstandard helmet use of marginal significance (P = .09). A strong interaction between motorcycle brand and cruiser style was observed (P < .001; Table 4). Among non-HarleyDavidson riders, cruiser riders were 3.1 times as likely as noncruiser riders to use a nonstandard helmet, whereas among Harley-Davidson riders, the likelihood of nonstandard helmet use did not differ significantly (RR = 4.71/5.46 = 0.86; 95% CI: 0.51–1.46; P = .585).
Discussion This study was conducted to obtain a snapshot of the helmet use patterns of motorcyclists in the SFBA—a major, heavily trafficked metropolitan region of California—and to identify rider characteristics associated with nonstandard, or bogus, helmet use. The SFBA is a motorcycling-intensive region of California; of the 9 counties that comprise the SFBA, 7 have per capita motorcycle registration rates higher than the California statewide rate and 2 have rates equal to the statewide
rate. (personal communication, Scott Masten, California Department of Motor Vehicles, December 2009; California Department of Finance 2009) Overall, 15 percent of surveyed motorcyclists reported wearing a nonstandard helmet sometimes or often. However, the prevalence of nonstandard helmet use differed widely across groups. Black riders and those with greater riding experience were more likely to wear nonstandard helmets. More important, motorcyclists who rode cruiser-style motorcycles were more than twice as likely to report nonstandard helmet use as riders of other motorcycles, after adjusting for brand. We observed that our cruiser RR estimate was confounded by motorcycle brand. This confounding appears to be driven by the Harley-Davidson brand. Harley-Davidson motorcycles were much more likely to be cruiser-style motorcycles and Harley-Davidson riders were much more likely to report wearing a nonstandard helmet. The cruiser RR unadjusted for brand was 3.3 and dropped to 1.9 after adjustment for brand. Reported use of nonstandard helmets varied greatly by brand. BMW riders had the lowest use of nonstandard helmets (5%) and Harley-Davidson riders had the highest use (51%). The magnitude of the estimated brand RRs is notable. Harley-Davidson riders were 11 times as likely to use nonstandard helmets as BMW riders. Three Japanese brands (Honda, Kawasaki, and Yamaha) had large and statistically significant associations with nonstandard helmet use. One common Japanese brand, Suzuki, was not significantly associated with nonstandard helmet use. The association between cruisers and nonstandard helmet use can be seen in the case of Honda and Yamaha, the Japanese manufacturers with, by far, the highest proportion of cruiser motorcycles in our sample—22 and 23 percent, respectively. Table 2. Number of motorcyclists and percentage reporting nonstandard helmet use by motorcycle make, San Francisco Bay Area Motorcycle Survey, 2008 Make BMW
Cruiser style Number % Nonstandard helmet
No Yes Total Harley-Davidson No Yes Total Honda No Yes Total Kawasaki No Yes Total Suzuki No Yes Total Yamaha No Yes Total Other No Yes Total Total No Yes Total
104 2 106 21 65 86 163 45 208 77 7 84 118 9 127 81 24 105 137 7 144 701 159 860
4.8 0 4.7 57.1 49.2 51.1 10.4 22.2 13.0 15.6 42.9 17.9 6.8 0 6.3 9.9 37.5 16.2 8.0 42.9 9.7 10.4 35.8 15.1
95% CI 1.6–10.9 — 1.5–10.7 34.0–78.2 36.6–61.9 40.1–62.1 6.2–16.2 11.2–37.1 8.7–18.3 8.3–25.6 9.9–81.6 10.3–27.7 3.0–12.9 — 2.8–12.0 4.4–18.5 18.8–59.4 9.7–24.7 4.1–13.9 9.9–81.6 5.4–15.8 8.3–12.9 28.4–43.8 12.8–17.7
154
Tsui et al.
Table 3. Risk ratios for nonstandard helmet use, San Francisco Bay Area Motorcycle Survey, 2008
Downloaded by [University of Auckland Library] at 17:33 19 October 2014
Characteristic
No. (%)
RRa
95% CI
P
Age (years) 18–24 62(7) Ref. — — 25–34 265(31) 0.79 0.28–2.24 .653 35–44 244(28) 1.10 0.38–3.22 .864 45 or older 257(30) 0.90 0.29–2.77 .849 Unknown 32(4) — — — Race/ethnicity White 604(70) Ref. — Black 14(2) 2.15 1.01–4.56 .047 Asian 76(9) 1.07 0.54–2.13 .845 Other 166(19) 1.52 0.92–2.51 .099 Hispanic heritage No 766(89) Ref. — — Yes 94(11) 0.93 0.54–1.60 .787 Motorcycle manufacturer BMW 106(12) Ref. — — Harley-Davidson 89(10) 11.04 4.20–29.06
