The surprising impact of helmets on safety

Abstract

Contrary to popular belief, helmets main impact on safety has not been the protection they provide, but the increased in the risk of accident associated with them. Helmets protect, but not enough to compensate for the increased risk of accident.

Although the motivation for wearing a bicycle helmet is to reduce the risk of death & serious brain injury, the net result of imposing a helmet law has been to increase the risk of death & serious injury.

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Two types of injuries

Broadly speaking, there are two types of injuries relevant to bicycle helmets:

  1. Minor injuries like bruises and lacerations to the skull.
  2. Severe injuries like brain injury, skull fractured, or neck injury that can lead to disability.
Bicycle helmets provide cushion against minor head injuries.  On hitting a flat surface, the polystyrene compresses to attenuate the impact.  However, in a serious impact, helmets tend to “fail”, or break into pieces, providing little protection.  A polystyrene based helmet is not designed to protect in a serious accident:

“In cases of high impact, such as most crashes that involve a motor vehicle, the initial forces absorbed by a cycle helmet before breaking are only a small part of the total force and the protection provided by a helmet is likely to be minimal in this context. In cases where serious injury is likely, the impact energy potentials are commonly of a level that would overwhelm even Grand Prix motor racing helmets. Cycle helmets provide best protection in situations involving simple, low-speed falls with no other party involved. They are unlikely to offer adequate protection in life-threatening situations.

soft-shell-helmet
A soft-shell helmet is a piece of polystyrene covered by a layer of plastic less than 1mm thick.  It can protect in a minor accident.  However, it is not designed to protect in a serious accident.
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Helmets make little difference in a serious accident, as Dr Hooper reports:

“Looking at evidence, it does not matter if people are wearing a helmet or not, any serious accident on a bike is likely to kill them,”

Wearing a helmet can induce cyclists to take more risks, sometimes with serious consequences:

“the increased use of bike helmets may have had an unintended consequence: riders may feel an inflated sense of security and take more risks. …

The helmet he was wearing did not protect his neck; he was paralyzed from the neck down. …

”It didn’t cross my mind that this could happen,” said Philip, now 17. ”I definitely felt safe. I wouldn’t do something like that without a helmet.” ”

While helmets can reduce minor head injuries, they can also increase the risk of neck injury.  Contrary to popular belief, helmets are not designed to protect against severe brain injury, and may aggravate it some circumstances.

Yet most  helmet studies fail to distinguish between minor and sever injuries.  Minor and severe head injuries are lumped together into a category called “head injuries”. The much larger number of minor injuries masks the trend in severe injuries. An apparent reduction in minor head injuries can mask an increase in severe injuries.

What does the data tell us?

Relevant data that can shed some light on this issue comes from New South Wales (NSW), Australia. It includes minor injuries, separated as head injuries and non head-injuries, for child cyclists before and after the helmet law. It also includes data for death & serious injuries for child cyclists and pedestrians.

Before the helmet law, about 20% of cyclists wore helmets. After the helmet law, about 80% of cyclists wore helmets. To make sense of the data, it is worth keeping in mind two key factors affecting injuries:
  1. There were 40% fewer cyclists after the helmet law. The risk must be adjusted per cyclist.
  2. The helmet law was introduced at the same time as other road safety measures, like a crackdown on speeding and drink driving. Injuries declined significantly for pedestrians, who face a similar risk as cyclists, being hit by motorists. By adjusting for safety improvements observed with pedestrians, we can isolate the effect of external factors, so that we can better understand what can be genuinely attributed to helmets.

A detailed analysis of the data can be found here.  Additional injury data for pedestrians can be found here.  Here is a short summary of the analysis of the data:

  1. Compared to what would have been expected without the helmet law, the risk of non-head injury for cyclist almost doubled. This indicates that the risk of accident almost doubled. Explanations for the increase in accidents include risk compensation and safety in numbers.
  2. Compared to what would have been expected without the helmet law, the risk of death & serious injury increased by 57%. This indicates that helmets did protect against some serious injuries, but not enough to compensate for the rise in accidents.
  3. Compared to what would have been expected without the helmet law, the risk of head injuries for cyclists increased by 40%. This indicates the helmets were more effective at preventing minor injuries.

risk_of_injury_after_helmet_law

Compared to what would have been expected without the helmet law, the risk of accidents almost doubled, the risk of death & serious injury increased by 57%, and the risk of head injury increased by 40%.

The data indicates that bicycle helmets do protect, but not enough to compensate for the rise in accidents.

Accident avoidance vs protection

Although helmets do protect, they have not protected enough to compensate for the rise in accidents.  The net safety effect of imposing helmet has been to increase the risk of  injuries, both head-injuries and non-head injuries, and both minor and serious injuries.

However, the sharp increase in the risk of accident is the most significant impact of helmets on safety. Why wear a “safety device” that increases the risk of accident while failing to protect sufficiently to compensate for this increased risk of accident?

There is an odd discrepancy between the motivation for wearing bicycle helmets (to reduce the risk of death & chronic disability), and the actual result of the helmet law: a much higher risk of death & serious injury.

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