No clear evidence from countries that have enforced the wearing of helmetsBMJ 2006; 332 doi: https://doi.org/10.1136/bmj.332.7543.722-a (Published 23 March 2006) Cite this as: BMJ 2006;332:722
Selection criteria, data sources, reliability of data
The statistical power of detecting effects of a helmet law is proportional to the change in percentage helmet wearing (%HW). If %HW increases by 50 percentage points (e.g. from 35%-85% of cyclists) and (as suggested by case-control studies) helmets prevent 68% of head injuries (HI), HI rates should fall by 46%. Changes this large would be evident in times series. The selection criterion for inclusion was therefore an increase in %HW of at least 40 percentage points in less than 1 year.
Medline/web searches were used to locate jurisdictions that passed helmet laws and determine changes in %HW. Table B provides details of jurisdictions where the required increase was not achieved. Published data were located for all other jurisdictions from the sources listed in Table A. Public hospitals in Australia and New Zealand have computerised recording systems based on the ICD coding system, enabling cycling injuries to be readily identified and separated into head and non-head injuries. Several hundred cyclists were admitted to hospital every year in every jurisdiction (Table A), so the statistics for percent head injury are considered reliable. As shown in Figs 1-3, B, and C, they exhibit smooth trends, rather than random fluctuations. The total of 10,479 head injuries (Table A) is an order of magnitude greater than the 1,196 head injuries (4 studies), 347 brain injuries (3 studies) and 90 serious head injuries (>AIS 2; 2 studies) from which combined odds ratios were derived in the frequently-cited review of bicycle helmet efficacy.
Data for emergency room treatments in Halifax, Nova Scotia are considered more problematical, because annual totals were not available (making it difficult to separate trends from the effect of legislation) and, even before legislation, there were only 15 head injuries (and 401 non-head injuries).
Bike/motor-vehicle collisions: reduced safety in numbers?
Other successful road safety campaigns commenced about the same time as the Australian helmet laws. Pedestrian fatalities fell by 43% (Victoria), 34% (NSW) and 22% (South Australia) in the first 3 calendar years of the helmet law, compared to the 3 years before legislation. Early analyses ignored the effect of such confounding factors.[w2] However, to fully assess the effects of helmet laws, cyclist injury rates should be compared with those of other road users such as pedestrians.
In Victoria, the Transport Accident Commission (TAC) funds all medical treatment for road injury. It was therefore possible to investigate ‘Safety in Numbers’, by comparing pre and post-law rates of death or serious head injuries (DSHI) and other serious injuries (OSI) to pedestrians and cyclists in collision with motor vehicles.[w19] TAC’s definition of DSHI was: death or hospital admission for skull fracture (800, 801, 803, 804) or intracranial injury (851-854) excluding concussion. OSI was defined as ICD9 codes: 599.7, 765.1, 802, 805-839, 860-869, 870.3, 870.4, 871, 878, 885-887, 895-897, 900-902, 940-957, 994.1, 994.7.
Table C compares DSHI as a percentage of all serious injuries (%DSHI) of pedestrians and cyclists. For cyclists, %DSHI fell by 1.7 percentage points to 24.8% in the 2 years after the law. For pedestrians, the fall over the same period was actually greater – 2.5 percentage points. Helmets are popularly believed to prevent death and serious head injury, yet the fall in %DSHI was actually greater for pedestrians than cyclists.[w19]
According to the ‘Safety in Numbers’ rule, injury rates per cyclist are lower when more people cycle. So, did the reduction in cycle-use because of helmet laws increase the risk per cyclist, relative to other road users? The road safety campaign, introduced at the same time as the helmet law in Victoria, reduced pedestrian DSHI to 74% of pre-law numbers (Table C). Cyclist DSHI fell to 57% of pre-law numbers, but there were fewer cyclists – only 69% as many as before the law (Table 1). DSHI should therefore have fallen to (69%x74%)=52% of pre-law numbers for cyclists to enjoy the same injury reductions as pedestrians. The actual fall suggests cyclists did not fare as well with the helmet law as they should have done without it. Increased injury rates following helmet laws was also noted for child cyclists in NSW.
A similar phenomenon was also noted for Australia as a whole. Comparing 1988 (before any helmet law) with 1994 (when all states had enforced laws) cyclist, pedestrian and all road user deaths fell by 35%, 36% and 38% respectively; head-injury deaths fell by 30%, 38% and 42%.[w20] Thus the decline for cyclists was less than other road users.[w20] This suggests that, given the fall in cycle-use, the risk of fatal injury per cyclist increased relative to other road users.
Problems with case-control methodology
Real-life head injury data, before and after helmet laws, contradict the results of case-control studies, where cyclists who choose to wear helmets tend to have fewer head injuries than non-wearers. However, helmet wearers often differ considerably from non-wearers in riding styles and attitudes to risk.
Case-control methodology can fail. We now accept that hormone replacement therapy (HRT) does not decrease (and may increase) the risk of heart disease. Yet a review of what were considered the best quality observational studies (11 case-control studies, 16 prospective studies, 3 cross-sectional studies) concluded that HRT decreased the risk by 50%. These misleading results (and others e.g. from vitamin supplement studies[w15]) were attributed to problems distinguishing between the effect of the treatment (e.g. HRT, or helmet wearing) and the other differences (e.g. socioeconomic status, attitudes to risk) between those who chose the treatment and those who did not. When there are substantial differences (known as confounders) between treatment and non-treatment groups, case-control studies can, and have, produced misleading results, so should be treated with caution
Helmet efficacy vs injury severity
Most of the case-control studies (and data from Halifax, Nova Scotia) used emergency department records. In contrast, Table C and the line drawings show injuries severe enough to warrant hospital admission. Changes in recording or admission policies are more likely to affect minor injuries, e.g. whether concussions are admitted for observation. A published analysis of hospital data for cyclists in bike/motor-vehicle collisions, reported that numbers with head injury (including wounds or concussion) fell by 48% and 70% in the first and second years of the law, compared to 23% and 28% for admissions without head injury.[w2] It was later noted that numbers of pedestrians with concussion fell by an equally-impressive 29% and 75% in the same years. Although the effect was cited as a major success of helmet laws, it probably had nothing to do with helmets.[w19]
The 36,500 injuries in the line drawings represent what actually happened when entire populations of several million cyclists were required by law to wear helmets. They represent all cyclist admissions in all public hospitals, including the most severe injuries. The case-control studies were of cyclists receiving treatment in emergency departments; the vast majority did not require hospital admission. In the 1996 Seattle study, 73% of head injuries did not involve concussion or other brain injury.[w21] Helmets undoubtedly prevent wounds but increase the size and mass of the head. They are designed to deform on impact, but a broken, dented shell may not slide easily on surfaces. This, together with greater torque from increased head size, may increase the risk of rotational injuries, such as diffuse axonal injury. In monkeys, linear accelerations of 665-1230g caused no detectable brain damage, but, in every case, rotational accelerations (348 to 1025g) caused concussion or other brain injury, including subdural haematoma, subarachnoid haemorrhage and intracerebral petechial haemorrhage.[w22] Angular accelerations in dummies wearing bicycle helmets were measured at 12 times the 4500 rads/s2 for onset of vein rupture.[w20]
For serious injuries (>AIS 2), the review of helmet efficacy derived odds ratios from only 2 studies and a total of 90 head injuries >AIS2. With such small numbers and major differences in behaviour and attitudes to risk between helmeted and non-helmeted cyclists, case-control studies are unable to provide reliable information on injuries of this severity, and cannot address other consequences of legislation, such as risk compensation or reduced safety in numbers. The line drawings show all consequences, so represent the most reliable information on helmet laws. The lack of response in %HI to substantial increases in helmet wearing, but substantial reduction in cycle use (Table 1, Fig C) indicate that the risk per cyclist increased relative to that for other road users. Thus the disadvantages of enforced helmet laws outweigh any benefits; it would be better to concentrate on other road safety interventions with few drawbacks and large, demonstrable benefits.
Non-helmeted cyclists have lower mortality rates than non-cyclists; countries where cycling is popular but few cyclists wear helmets have much lower fatality/injury rates per km cycled than those with higher helmet wearing rates.[w17] Governments should therefore provide appropriate facilities to encourage more people to cycle for the health and environmental benefits and make cycling even safer because of ‘Safety in Numbers’.
- Provenance of article
The author is a commuter cyclist who, in association with the Bicycle Federation of Australia, investigated the effect of helmet legislation. She works at the University of New England and has a first class honours degree in mathematics and a PhD in statistics/genetics. Experience in statistical modelling, especially how biased and misleading results can be obtained from fitting incorrect or inappropriate models, motivated her to investigate the contradiction between predicted benefits from case-control studies but minimal changes in head injury rates when helmet wearing increased substantially because of legislation.
- Additional References
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Table A Dates of helmet laws, sources and descriptions of injury data, helmet wearing surveys and other road safety initiatives introduced at the same time as helmet laws
Description of head and other injuries (HI and OI), surveys and other safety initiatives
South Australia (SA); Marshall and White Helmet law 1/7/91
HI: (Fig 1) = hospital admission with principal diagnosis of skull fracture (800-802.1, 802.4-804.06) intracranial injury (850-854.16) or wound to the head (870-871, 873-873.39, 873.42, 873.52); concussions (850) were tabulated separately. OI = other admissions.
Numbers: 1637 cyclist head injuries and 4170 total admissions over 6 years (1988-93).
Surveys: %HW 49% (1991) increased to 97% (1992) and 98% (1993) in commuter cyclists (approx. 2,000/year) entering the city of Adelaide on a weekday. Self reported %HW increased 15%-91% (³ 15 years) and 42%-84% (<15s) according to surveys (1990 & 1993 of 3,000 households).Other safety initiatives: 3 calendar years post-law had 33% fewer pedestrian fatalities/serious injuries (201, 207, 177) than the 3 calendar years pre-law (272, 291, 307). Fig A shows total road casualties by month in relation to the start of the helmet law.
Western Australia (WA); Hendrie et al. Helmet law 1/1/92
HI: (Fig 2) = all hospital admissions with ICD9-CM injuries coded to the AIS body region of head (1998 onwards). For 1979-87, HI = admission with skull fracture (800-804), intracranial injury (850-854), open wound to head (873.0-873.1), injury to blood vessels of head or neck (900) or injury to optic or other cranial nerves (950-951). To 1978, HI = IDC8 codes 800-804, 850-854, 873, 904, 950-951. OI = all other admissions.
Numbers: 1961 head injury, 7480 total cyclist admissions in the 10 years 1989-98.
Surveys: Approx. 3000 commuter, recreational, primary and secondary school children observed per year. Pre-law %HW 39%, post-law 81%. Healthcare savings substantially less than the estimated cost of $20 million for purchasing helmets in WA (pop. 1.6 million, 1991).
New Zealand; Robinson Povey et al.,[w4] Helmet law 1/1/94
HI: (Fig 3) = admissions with skull fracture (800–804), concussion or intracranial injury (850–855); OI = admission with limb fracture (810–829); excludes accidents involving motor vehicles.
Numbers: 730 (adult) and 779 (primary schoolchild) head injuries, 1477 (adult) and 1838 (primary schoolchild) total admissions over the 7 years 1990-96. Surveys: %HW (shown in Fig 3) from morning and afternoon school and commuter ‘rush hour’ at 60 sites in towns and suburbs (approx. 10,000 cyclists/year).
New South Wales (NSW); Robinson Adult law 1/1/91; <16 years 1/7/91
HI: (Fig B) = hospital admissions with head injury (NSW Health classification); OI = admission with other injury; cyclists with both head and other injuries were included in both categories.
Numbers: 1796 (child) and 1067 adult head injuries out of 6391 child and 3838 adult admission over 5 years (1989-93).
Surveys: (Table 2): observational sites covered metropolitan and regional areas: 59 schools, 39 road intersections and 22 recreational areas.[w13] The same sites, times and (if possible) the same observers were used at the same time of year (April 1991, 92 and 93) in similar weather, to make all 3 surveys as comparable as possible. Law for adults commenced before the 1991 survey, so effect of legislation on adult cycle-use could not be estimated, but pre-law %HW was available from an earlier survey (September 1990).
Other safety initiatives: 3 calendar years post-law had 34% fewer pedestrian fatalities than 3 calendar years pre-law.
Carr et al. Helmet law 1/7/90
HI: (Fig C) = any hospital admission with skull fracture (800-803), concussion/intracranial injury (850-854), open wound of ear or head (872, 873.0, 873.1, 837.8, 873.9).
Numbers: Approx. 2509 head injuries, 8305 total cyclist admissions over 7 years (Jul 1987-June 94).
Surveys: (Table 2): 64 sites observed for two 5-hour periods chosen from 4 time blocks of weekday morning, weekend morning, weekday afternoon and weekend afternoon, a total of 640 hours.[w1] Identical surveys in similar weather in May 1990, 1991 and 1992.[w1]
Other safety initiatives/trends: 3 calendar years post-law had 43% fewer pedestrian fatalities than the 3 calendar years pre-law. 29% and 75% reductions in numbers of pedestrians with concussion in 1st and 2nd years of the helmet law.[w19]
Halifax, Nova Scotia, Canada; Leblanc et al.[w7] Law 1/7/97
HI: = concussions, lacerations, dental injuries or other head injuries causing death or requiring follow-up, observation in the emergency department, admission to hospital or transfer to another health facility. OI = all treatments at the health centre except those listed above.
Numbers: HI accounted for 15/416 (3.6%, 1995/96), 3/222 (1997, 1.4%) and 7/443 (1998/99, 1.6%) of injuries respectively (p=0.06). Numbers of child cyclists with HI admitted to Nova Scotia’s hospitals in the 3 pre-law years (29, 23, 7) increased to 13 in 1997/98, the year helmets became compulsory.[w7]
See text and Table B for formal selection criteria and jurisdictions where helmet wearing failed to increase by at least 40 percentage points.
Table B Jurisdictions not included because percent helmet wearing (%HW) increased by less than 40 percentage points
All-age helmet law was introduced 1/7/91, but not enforced for 18 months; %HW increased temporarily, then declined until penalties imposed.[w23]
Non-enforced law for child cyclists introduced October 1995. HW increased from 45% pre-law in 1995 to 68% in 1996 and 1997, then returned to pre-law levels.[w10] %HI trended down smoothly from 94/95 to 97/98; the largest fall (from 33.9% in 96/97 to 28.5% 97/98) was when %HW was static or declining.[w24] Despite the decline in %HW from 1997 onwards, %HI continued to decline, falling by 26% from 1997/98 to 2001/02, non-HI by 12%.[w25]
British Columbia, Canada
All-age helmet law introduced in September 1996. %HW increased from 51%-74% in metropolitan areas, 32%-58% elsewhere. %HW of 6-15 years olds increased from 35%-61%. Declining trend in children’s %HI; the largest fall (7.4 percentage points, from 39.9%-32.5%) was from 1994/95 to 1995/96, a year before legislation.[w24]
New York State
Non-enforced law introduced June 1994. In Brooklyn %HW decreased from 5.6% to 4.2% with legislation. In Queens, education plus legislation increased %HW from 4.7% to 13.9%.[w26]
Children’s law from 1997. Legislation increased %HW of 7-12 year old boys from 5.6% to 20.8% in Brownard.[w27] %HW in a survey of elementary school children was 79% in counties with helmet laws, compared to 33% in non-law counties.[w28] In Hillsborough county, %HW increased from 14% in 1996 to 57%, 67%, 56% and 50% in 1997, 1998, 1999 and 2000. No head injury data, but number of child cyclists (0-14 years) injured in motor vehicle accidents declined (135, 125, 140, 125, 100, 77, 101, 87 in 1993-2000)[w29]. No mention of any attempts to estimate changes in cycle-use, so the fall in injuries may have been due to reduced cycling.
State helmet laws (for children under 16) increased average probability of helmet use by 18.4%.[w30]
Table C TAC (Transport Accident Commission) data for average numbers of deaths and serious head injuries (DSHI) and all serious injuries (ASI) per year in Victoria (from Robinson[w19])
Injuries due to collisions with motor
vehicles (average number per year)
Post-law as % of pre-law
Adjusted for 30% fall in cycling
*DSHI as defined by TAC (death or hospital admission with skull fracture (800, 801, 803, 804) or intracranial injury excluding concussion (851-854). OSI = other serious injury (599.7, 765.1, 802, 805-839, 860-869, 870.3, 870.4, 871, 878, 885-887, 895-897, 900-902, 940-957, 994.1, 994.7). ASI = DSHI + OSI.
- Correction Published: 06 April 2006; BMJ 332 doi:10.1136/bmj.332.7545.837-a
- PaperInjury patterns in cyclists attending an accident and emergency department: a comparison of helmet wearers and non-wearersPublished: 11 June 1994; BMJ 308 doi:10.1136/bmj.308.6943.1537
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