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5 Helmets and Safety
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5.1 Helmet Use
Many cyclists use helmets for extra protection, and in some states, helmets are mandatory, either for all cyclists or for cyclists under a specified age. Helmets can reduce the head-injury rate, but are not a replacement for good judgement and good practices. It is worth noting that many collisions do not involve motor vehicles, but instead result in simple falls. Helmets are particularly effective in preventing head injuries in such collisions, provided the helmet covers the area that is hit. It is also worth noting that experience and good bike-handling skills can substantially reduce the number of such collisions.
5.2 Helmet Effectiveness
The basic rule is to have a much higher opinion of helmets when buying one or deciding to wear one than when actually riding a bicycle: it may be prudent to use a helmet, but it is not prudent to take extra risks simply because you are wearing a helmet.
The ANSI and SNELL standards for helmets require that the helmet provide reasonable protection against a fall of 1 to 1.5 meters, at which point your vertical velocity will be 10 to 12 mph. The direction isn't particularly important, but the speed is: a helmet has a good chance of preventing a head injury only in impacts at speeds of up to 10 to 12 mph. Above this speed, the helmet's role changes to one of head-injury reduction rather than head-injury prevention. You should note, however, that impact speed is not the same as bicycle speed and if the helmet does not take up the initial impact, the impact that the helmet has to absorb will be less.
While serious injuries can occur regardless of speed, to a first approximation, you may assume that a helmet's effectiveness drops with increased speed, giving good protection at low speeds, moderate protection at medium speeds, and relatively little protection at high speeds. This is illustrated in the following figure, where green indicates relative safety, red indicates high risk (almost certainty of a head injury at some level, and yellow indicates the transition region).
The ranges in the figure are meant to be suggestive only, and not represent hard boundaries. While the higher speed collisions may raise some concerns, it is worth noting that the majority of collisions are low speed ones. Furthermore, there is at present (c.a. 1999 in the U.S.), considerable room for improvement in cyclist behavior, and in fact most of this document describes how cyclists can decrease their risks, and reduce collision speeds in collisions that are unavoidable.
The Snell Memorial Foundation has a study of helmet effectiveness available on its web site, but does not guarantee that the URL will not change at some point. The study included some interesting observations, including the following:
- helmet fit is critical. A poorly fitted helmet can reduce the helmet's effectiveness by a factor of 2 or 3.
- the forehead is a particularly likely impact point (so it is important to keep this area covered and not cant the helmet backwards to be stylish).
- collisions can be sufficiently severe that a helmet will not prevent a fatality, although helmets can be effective in less severe collisions.
- educational programs and other safety measures are also important.
It is also worth noting that the study cited above was performed in an affluent area, where better educational programs may have been available than in financially disadvantaged areas. For reasons described below (how better behavior on the part of cyclists can reduce the impact in collisions), such educational programs can have a large impact on helmet effectiveness.
In any event, when purchasing a helmet, it is worth paying more to obtain better advise on how to fit the helmet properly if you do not have the experience to make a good selection oneself.
5.3 Safety Considerations
Given the limitations imposed by helmet design (as indicated by the standards), there are several points to consider:
- at 15 mph, the difference in impact speed in a collision is 30 mph higher if you ride the wrong way than if you ride in the same direction as other traffic, assuming a collision with a vehicle going parallel to the cyclist.
- a car can lose nearly 15 mph for each second of braking. In most cases, both a car and a bicycle will do something to try to avoid a collision, even if that something is not always 100 percent effective. Techniques that provide even one or two seconds of additional time for a cyclist or motorist to react can make a big difference in the outcome of an accident.
- Heads are not the only things that can be injured, and eliminating head injuries will not prevent all fatalities.
- In a collision with a vehicle, some parts of the vehicle are worse to hit than others. The supports that hold up the roof in particular have to be strong enough to prevent the roof from collapsing if the car flips over, and have to be narrow enough so that the driver can see adequately. Hitting one of these supports will concentrate the impact on one part of a helmet, and the helmet will not be as effective as it would be when hitting a flatter object.
The techniques described in the previous sections of this document are geared towards preventing collisions, primarily by riding predictably and in such a way that all parties have more time to react and slow down. Even when collisions occur, the impacts will on the average be less severe than they would be otherwise. Given the limits imposed by helmet design, good technique can go a long ways towards keeping unavoidable accident in the speed range where helmets can either prevent or substantially mitigate head injuries.
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