Is it true that cycling in the Netherlands without a helmet is safer, then cycling with a helmet in the USA?
Most likely it is, given the status of cycling in those countries and the ridiculously weak helmet standards.
Cycling has safety in numbers: the more cyclists there are, the safer every one of them is. Netherlands has a lot of cyclists.
Also the risk of cycling one kilometer is about the same as the risk of walking one kilometer as a pedestrian. If you consider a country dominated by the automobile, to get anywhere from the suburban area you have to ride a huge distance. If you consider a country dominated by cycling, the distances to get to places of interest are probably much shorter. Thus, I assume a random trip in Netherlands is probably much shorter than a random trip in USA.
The additional risk of cycling compared to risk of walking comes from the possibility to cycle far longer distances than it is reasonable to walk.
It is not a surprise that cycling is not more dangerous than walking is. Both are means of transport where the head is at about the same height and there's a danger of falling.
About the head height: helmet tests are done with a severed head. If we consider a cyclist or pedestrian, the head is at about 1.8 meter height. However, when falling, relatively more energy is in the head which moves fastest than your toes have. The toes when falling won't move much, but the head is moving at a great speed. Thus, falling is not comparable to dropping a severed head at 1.8 meter height. When considering the physics of a falling cyclist, it can be modeled as a rod 1.8 meters long rotating about one end. Solving the equations, one finds that the head of the rod hits the ground with as much speed it would have if a severed head was dropped from 1.5 times its height, i.e. 2.7 meter height instead of 1.8 meter height.
So, we can establish that a good helmet test drops a severed head from 2.7 meters height. Let's take a look at the helmet tests:
- The EU test (EN 1078) drops a severed head from 1.5 meter height
- The USA test (CPSC) drops a severed head from 2.0 meter height
- The best test (Snell) drops a severed head from 2.2 meter height
Not a single test drops a severed head from 2.7 meter height.
A cyclist on the road faces two threats:
- The cyclist's head can hit an oncoming motor vehicle. The speed of this impact is so large that no helmet can protect from it.
- The cyclist can fall and the head can impact pavement. This is the impact that has the same energy as a severed head dropped from 2.7 meter height. The existing helmet standards are inadequate for protecting against this.
Based on this, we can expect that helmets do not do much to improve the safety of cyclist. The difficulty of assessing whether helmets prevent head injuries is that severe head injuries are so rare that the only reasonable form of test is a whole-population test where a mandatory helmet law is suddenly enacted and enforced, so that the rate of helmet wearing suddenly goes from negligible to significant. Then we need some measure of severe injuries. As any measure of "severe head injury" is very hard to define, the easiest measure is to count the number of cyclists dead resulting from accidents and assume that the rate of severe head injury is proportional to this.
Such a mandatory helmet law has been at least enacted and enforced in New Zealand. By looking at the statistics, one can see that it can be the case helmets reduced fatalities. However, the argument is more against helmet laws than for helmets, because of two reasons:
The reduction of fatalities is nowhere comparable to the reduction of fatalities due to for example car seatbelts. Often times, a helmet is compared to car seatbelts, claiming that both are very effective safety devices. This is false: only one of them is a very effective safety device.
Due to the mandatory helmet laws, cycling decreased so it is possible that the fatalities divided by number of active cyclists actually stayed the same.
It is a good question whether a helmet could be made that bicyclists are willing to wear and that at the same time is good enough to be an effective safety device. Bicyclists need ventilation unlike motorcyclists that produce their power in an internal combustion engine. Bicyclists are limited in power so aerodynamics and light weight are important. Thus, bicycle helmet must be lightweight, aerodynamic and keep the cyclist's head cool. These design features are at odds with the safety features of a helmet.
In addition to these whole-population tests for which we can thank New Zealand at least, there are scientific studies where cyclists seeking healthcare services are assessed for head injuries. The problem is that to get a large enough sample size for head injuries, the term "head injury" must be defined such that a simple bleeding wound in the head is a head injury. (Then the result is obviously that helmets protect against head injuries if a "head injury" is a bleeding wound.) Also, there is fundamental flaw in this test that the cyclists more worried for their safety (and thus wear a helmet) visit healthcare due to even minor suspicions, whereas the cyclists that aren't worried for their safety (and thus won't wear a helmet) visit healthcare only if they truly have a serious head injury. (Then the result is obviously that helmets appear to protect against head injuries but that result was only due to a flawed scientific study.)