The analytic framework (Appendix Figure 1, available online) shows the postulated mechanism through which motorcycle helmet laws affect incidence and severity of non-fatal and fatal injuries. UHLs can lead to increased helmet use, resulting both in reduced incidence and severity of non-fatal injuries and in reduced fatal injuries. If motorcycle helmet laws affect overall motorcycle use, as some have speculated,⁹ that could also contribute to observed decreases in fatal and non-fatal injuries. Other factors that may influence helmet use or injury include strength of the law (UHLs versus PHLs); intensity of enforcement efforts; type of helmet used (U.S. Department of Transportation approved or non-approved); and individual attitudes such as the desire not to wear a helmet.

Reviewers from the George Institute in Sydney, Australia, conducted the search for evidence, and the detailed search strategy can be found at: www.thecommunityguide.org/mvoi/motorcyclehelmets/supportingmaterials/SShelmetlaws.html. Briefly, databases relevant to health or transportation were searched from database inception to August 2012. Reference lists of reviews and reports relevant to the current review were also searched. Two reviewers from the George Institute performed the initial screening and eliminated publications not evaluating motorcycle helmet laws. Reviewers from CDC’s Community Guide Branch further screened the publications using the predetermined inclusion criteria listed below.

Studies were included in the current review if they evaluated motorcycle helmet laws and also met the following criteria:

A total of 262 potentially relevant articles were identified in the search for evidence, and 125 were candidates for inclusion. Forty-nine articles were excluded as they did not meet the inclusion criteria: Three papers^11–13 were not published in English, 11 papers^14–24 evaluated helmet laws in low- or middle-income countries, 20 papers^25–44 were not primary evaluations, and 15 papers^45–59 did not report on the outcomes of interest. Overall, 71 studies^60–130 with 78 study arms were included in the current review (Figure 1), with five studies^131–135 providing additional information on already included studies. Of the 71 included studies, 60 studies^{60–66,68–72,74,76–79,81–90,92–106,108–116,120–122,125–130} with 67 study arms evaluated helmet laws in the U.S. As mentioned above, this paper is solely focused on the U.S., and only U.S. data are reported in the following sections.

Detailed assessment results can be found on the Community Guide website (www.thecommunityguide.org/mvoi/motorcyclehelmets/supportingmaterials/ROB-hel metlaws.pdf). All included studies were observational studies and no study performed blinded assessment of primary outcomes. Included studies obtained data from routine government and hospital reports, which were unlikely to be affected by helmet laws or law changes. Most included studies examined helmet laws or law changes that were independent of other traffic safety interventions.^{60–66,68–71,74,76–79,81–90,92–97,99–106,108–116,120–122,125–130} Some studies did not provide sufficient data points for reliable statistical inference,^{92,113,114,130} had missing data,^62,72,92 or did not describe statistical methods.^{72,87,88,92,94,95,98,111,120,122,125,126,130} Three studies^99,122,126 reported observed helmet use without describing study methods; outcome reliability could not be assessed.

Sixty studies^{60–66,68–72,74,76–79,81–90,92–106,108–116,120–122,125–130} with 67 study arms evaluated motorcycle helmet laws in the U.S. These studies either evaluated law changes such as helmet law implementations (from no or partial laws to UHLs)^{60,71,74,78,88,89,94,95,99,101,103,104,109,116} or repeals (from UHLs to partial or no laws),^{61,63,66,70–72,81–83,85,87,90,92,93,96,98,100,105,106,108,111,116,121,122,125–128} or compared the impact of UHLs to partial or no helmet laws.^{62,64,65,68,69,76,77,79,84–86,96,97,102,106,110,112–115,120,129,130} Study designs included in the review were panel,^{64,70,71,76,77,79,81,84–86,102,112,115,128} time series or before–after with concurrent comparison groups,^{62,65,66,96,97,110,114,120,126,129} interrupted time series,^{61,74,78,103,105,121} uncontrolled before–after,^{60,63,72,82,83,87–90,92–96,98–101,104,106,108,109,111,116,122,125,127} and cross-sectional.^{68,69,106,113,114,130} More detailed information can be found at: www.thecommunityguide.org/mvoi/motorcyclehelmets/supportingmaterials/SET-helmetlaws.pdf.

Twenty-two studies^{60,63,65,68,69,78,82,83,87,89,90,95,96,99,101,104,108,110,113,125,127,129} of the 60 from the U.S. reported population characteristics. The study population consisted of motorcycle riders and passengers observed for helmet use¹²⁵ or who sustained fatal or non-fatal injuries during motorcycle crashes.^{60,63,65,68,69,78,82,83,87,89,90,95,96,99,101,104,108,110,113,127,129} Mean age of the study population was 36.5 years^{60,63,65,68,69,78,82,83,87,89,95,96,101,104,108,113,127} and a median of 91% were male.^{60,63,65,68,69,78,82,83,87,89,90,95,96,99,101,104,108,110,113,125,127,129}

Impact of helmet laws was assessed through the following outcomes: helmet use, motorcycle crash-related fatal and non-fatal injuries, and injury rates. These outcomes were assessed and reported in three categories:

The included studies reported many outcomes, almost all indicating substantial benefits associated with UHLs when compared to partial or no helmet law (Table 1). In the presence of UHL, there was higher prevalence of helmet use (Appendix Figure 2, available online); fewer fatal (Figure 2) and non-fatal injuries (Appendix Figure 3, available online); and lower injury rates. In the absence of UHL, there was lower prevalence of helmet use (Appendix Figure 4, available online); greater fatal (Figure 3) and non-fatal injuries (Appendix Figure 5, available online); and higher injury rates. Head-related fatal (Figures 2 and 3) or non-fatal injuries (Appendix Figures 3 and 5, available online) were especially affected by presence or absence of UHLs.

As of 2016, a total of 47 states in the U.S. had either UHLs or PHLs. The team performed additional analyses specifically to compare the laws’ effectiveness; results are summarized in Appendix Table 1 (available online). Results are similar to the overall findings that compared UHL to partial or no helmet laws (Table 1); states with UHLs, when compared with states with PHLs, have much higher helmet use and fewer fatal and non-fatal injuries.

The PHLs apply only to certain motorcycle operators. As of 2015, all 28 PHL states covered motorcyclists under a certain age (usually ≤21 years).⁶ The team summarized youth-specific data (Table 1) to determine if PHLs protect this population; the results are described below.

This body of evidence included a wide range of study designs. Even though each design comes with unique risks of bias, effect estimates across multiple study types, population groups, and outcome measures were remarkably consistent within the context of this review, and with independent estimates of efficacy of helmet use,³ demonstrating robustness of findings.

Total motorcycle-related fatal or non-fatal injuries are widely used measures of helmet law effectiveness. These total injury counts, however, are affected by the amount of motorcycle use (“riding exposure”), which could change in response to the presence or absence of UHLs. Many included studies attempted to account for driving exposure by dividing total counts of fatal and non-fatal injuries by the number of registered motorcycles, traveled vehicle miles, or crashes. Regardless of the specific measure used, UHLs were shown to be more effective than PHLs or no law in reducing fatal and non-fatal injuries.

The current review focused on motorcycle helmet laws in the U.S. Some of the included studies performed stratified analyses based on certain demographic characteristics. Evidence showed that UHLs were effective for male and female motorcyclists in increasing helmet use,^90,96,99 decreasing fatal and non-fatal injuries,^81,90,127 and decreasing fatalities per crash.⁹⁰ Compared with motorcycle operators, passengers usually had a lower prevalence of helmet use irrespective of the helmet law,^90,94,99,106 though implementing UHLs increased helmet use^94,99 and reduced fatal injuries^88,89,99 for both operators and passengers. When UHLs were repealed, passengers experienced greater decreases in helmet use⁹⁰ and greater increases in total fatal injuries and fatal injuries per crash.⁹⁰ Two studies compared helmet law effectiveness in rural versus urban areas and found that implementing UHLs reduced fatal injuries in both settings⁶⁰ and repealing UHLs increased fatal and non-fatal injuries in urban settings.¹²²

The UHLs were effective across age groups in increasing helmet use^{90,96,99,106,125} and decreasing overall fatal injuries^81,90,95,125 and fatal injuries per crash.⁹⁰ Young motorcyclists, when compared with their older counterparts, experienced larger decreases in fatal injuries when UHLs were implemented⁹⁵ and larger increases in total fatal injuries and fatal injuries per crash when UHLs were repealed.^90,125

No additional benefits of motorcycle helmet laws were identified in the included studies or in the broader literature.

Although one of the postulated harms associated with helmet use is increased risk of neck injuries, the ten included study arms^{62,68,69,82,89,99,103,104,109,127} that assessed this outcome found that fatal and non-fatal neck injuries accounted for a very small proportion of motorcycle-related injuries (median, 1.8%; interquartile interval, 0.2%−3.2%) and the type of helmet law had no noticeable effect on neck injury prevalence. One study arm found that implementing a UHL resulted in a reduction of 0.5 pct pts in neck injury-related fatalities.⁹⁹ Studies reporting non-fatal injuries found little difference in the prevalence of neck injuries between states with UHLs and states with PHLs or no law (median, −0.6 pct pts; range, −0.6 to 0.1 pct pts),^62,68,69 and minimal changes in prevalence of neck injuries when UHLs were repealed (0.1–0.2 pct pts)^82,127 or implemented (median, 0.0 pct pts; range, −0.3 to 0.6 pct pts).^{89,103,104,109}

Other postulated harms of helmet use include hearing or vision impairment, though evidence from laboratory and field research does not show much support for these claims.¹⁴¹ Finally, some researchers have raised concerns about risk compensation, postulating that riders wearing helmets feel safer and increase their risk-taking behaviors (reviewed by Hedlund¹⁴² in 2000). Evidence on this issue is limited, though authors of one study analyzed data from on-scene, in-depth investigations of motorcycle- related crashes in Los Angeles and concluded that helmet use was not associated with riskier behaviors.¹⁴³

Although substantial evidence shows UHLs are effective across population groups and settings, research gaps remain. Future studies could examine the role of enforcement on helmet law effectiveness, particularly in regard to the use of unapproved helmets.

More research is needed to better understand the impact of helmet laws on riders of low-powered motorized cycles (e.g., scooters, mopeds) that have been gaining popularity, especially in urban settings. In 2016, all types of low-powered cycles were covered in 12 of 19 states with UHLs and 11 of 28 states with PHLs; the remaining states with helmet laws covered motorized cycles above certain thresholds, such as engine displacement greater than 50 cc or those designed to go faster than 30 mph.⁶