The Safety in Numbers Effect


Does the chance of a cyclist being involved in a fatal accident increase as the number of cyclists on the road increases? The answer may surprise you. While the raw number of cycling fatalities does increase as the number of cyclists on the road increases, the chance that any one of those cyclists is killed is likely to decrease. This negative correlation between the number of cycling fatalities and the number of cyclists on the road is called the safety in numbers effect.

Safety in mumbers

The graph above illustrates the safety in numbers effect by plotting the number of kilometers cycled per inhabitant along with the number of cycling fatalities for every billion kilometers traveled for various countries. The graph is from a research report published by the OECD (Organisation for Economic Co‑operation and Development) titled “Cycling Health and Safety.” The safety in numbers effect generally holds for pedestrians as well as cyclists.

OECD_LOGO_1The OECD report cautions against thinking that the negative correlation seen in the graph leads to the conclusion that increasing the number of cyclists causes a decrease in the likelihood of cycling fatalities. The report also mentions that there has not been very much research into the possible causes of the safety in numbers effect and goes on to suggest several factors that may be involved.

  • Awareness: The more cyclists there are on the road, the more drivers will be aware of them. The more aware drivers are of cyclists, the less likely they are to hit them.
  • Expectancy: The more cyclists there are on the road, the more drivers expect to see cyclists. The more drivers expect to see cyclists, the more likely they are to actually see them and avoid hitting them.
  • Collective vigilance: The more cyclists there are on the road, the more likely it is that potentially dangerous or threatening situations will be noticed by at least one of them. Those who notice potential threats will communicate this information to the other cyclists who then have a greater chance to avoid the threat.
  • Knowledgeable leaders: The more cyclists there are on the road, the greater the chance that at least one of them will be knowledgeable about route and traffic conditions. The knowledgeable cyclist may lead the others along safer routes.

It may also be the case that safety and the number of kilometers ridden are linked in a causal loop. The safer cycling is, the more people are likely to cycle, and the more people cycle, the more opportunity there is for awareness, expectancy, collective vigilance or knowledgeable leaders to have an effect in reducing fatal accidents.

Hovenring 3

The Hovenring in the Netherlands is the world’s first suspension bridge designed to allow cyclists and pedestrians to safely cross a busy highway intersection.

Another factor that almost certainly plays a role in both increasing the number of kilometers ridden and in reducing fatalities is the presence of a well-developed cycling infrastructure. The Netherlands, Denmark, and Germany, the three countries with the best fatalities to kilometers ridden ratios shown in the graph, also have exceptionally well developed cycling infrastructures.  The Netherlands and Denmark are especially notable in this regard. The United States, on the other hand, has generally lagged behind the rest of the developed world in building well-designed and well-maintained cycling infrastructure.  The better the infrastructure, the more people are likely to use it to cycle safely.


Dutch cyclists

There is an additional factor to consider that is highlighted by the Netherlands which has by far the lowest ratio of fatalities to kilometers traveled of any of the countries shown in the graph. In addition to having an excellent cycling infrastructure, the Netherlands has a long-established cycling culture. As early as 1911 the Netherlands had more bicycles per capita than any other European country. When privately-owned cars became more affordable after Word War II, cycling became less popular as a means of transportation. As the safety in numbers effect would lead you to expect, this was accompanied by an increase in cycling fatalities. During the 1970s widespread demonstrations took place in the Netherlands protesting the number of child cyclists who were killed on the road. The government responded by restricting the use of motorized vehicles in cities and towns, building cycling infrastructure, and embarking on a program of safety education for both cyclists and drivers that placed the Netherlands at the forefront of countries that make serious efforts to incorporate cycling into people’s daily lives. Children in the Netherlands are taught how to cycle safely from a very young age; adults are tested on their ability to share the road with cyclists as part of the process of getting a Dutch driver’s license. According to a press release from the Netherlands Ministry of Transport, Public Works and Water Management, in 2004 the Netherlands was the only European country in which there were more bikes than people, and in 2007 26% of all trips made in the Netherlands were made by bike. As a society the Netherlands has embraced a culture of cycling and this has played an important role in producing both the very large number of kilometers traveled by bike per inhabitant and the very low number of cycling fatalities shown in the graph.

Any or all of these factors – a strong social and cultural history of cycling, the presence of an excellent cycling infrastructure, driver awareness and expectation, cyclist vigilance and leadership – may have a role to play in explaining the safety in numbers effect. The negative correlation between fatalities and the number of kilometers ridden is simple and easy to see, the causal factors that produce this correlation are complex and difficult to tease apart.

(This article has been cross posted to The Info Monkey.)

Cycling and Weight Loss Part 2: Metabolic Homeostasis

scale_caloric_balanceThis is the second in a series of posts about losing weight on the bike. Throughout this discussion it’s important to keep in mind that eating has many consequences for health, athletic performance and weight loss.  The “best” diet for losing weight is unlikely to be the “best” diet for maintaining either your health or a high level of athletic performance.

In the first part of this series, Riding the Bike to Lose Weight, I pointed out that there is a modifying factor that affects the basic  relationship between caloric intake and weight loss.  The basic relationship is that if you ingest fewer calories than you burn during the course of a day, you will lose weight, if you ingest more than you burn, you will gain weight, and if caloric intake and caloric burn are about equal, your weight will remain stable.  The modifying factor is metabolic homeostasis.

If you’re not familiar with the jargon, “metabolic homeostasis” is incomprehensible and useless gobbledegook.  In the context of thinking about weight loss, “metabolic” refers to the chemical processes that are involved in the breaking down of food in the digestive system and the ways the results of that breakdown process, such as glucose, are used by the body.

A homeostatic system is one that acts to keep itself at an equilibrium point or within an equilibrium range.  A common example is the climate control system in your house or car.  You set the thermostat for a high and a low temperature and the climate control system keeps the temperature of your house or car within this range.  If it gets too hot, the air conditioning is turned on; if it gets too cold, the heat comes on.

The human body is a brilliant homeostatic system in a number of ways.  If the core temperature of the body gets too hot, you sweat to rid yourself of excess heat; if core temperature gets too cold, you shiver to generate more heat.  If blood glucose drops too low, the system reduces glucose uptake at the muscles to maintain glucose supply to the brain.  Cyclists in the heat of battle sometimes wish it didn’t work this way as they go into a bonk and their legs shut down.

caloric homeostasisMetabolic homeostasis refers to the body’s mechanisms for maintaining a balance between caloric intake and caloric burn.  This homeostatic system is more complicated than previously thought and much about it is currently not well understood.    I’ll try to summarize some of the issues that come into play when considering weight loss, exercise and metabolic homeostasis.

One thing that appears to be soundly supported by the available evidence is that the body adapts to a regular, sustained change in the relationship between caloric intake and caloric burn by reducing the number of calories needed to fuel the same amount of activity.  Here’s an example of how this works.  Suppose your caloric intake and burn are balanced; on a typical day your regular activities burned 2000 kilocalories and you ingested about 2000 kilocalories in food during the day.  Your weight would remain stable.  Then you go on a diet and ingest only 1800 kilocalories a day.  At first you would lose weight because the 1800 kilocalories you ingest is less than the 2000 kilocalories you burn each day.  However, if you stayed on this 1800 kilocalorie per day diet for a period of time, your body would adapt to that reduced caloric intake by enabling you to engage in the same activity you were doing every day before you began the diet while only burning 1800 kilocalories.  Once that happens caloric intake and caloric burn are balanced again and you stop losing weight.

Reduction-in-RMR-GraphA slightly more technical way to express this idea is that the basal metabolic rate will change to maintain metabolic homeostasis.  Roughly speaking, basal metabolic rate is the rate at which kilocalories are burned to support normal daily activity.  When the balance between caloric intake and burn is disrupted through dieting or exercise weight is initially lost because fewer kilocalories are ingested than are burned and the basal metabolic rate has not yet adapted to the change.  However, after a period of sustained dieting or exercise the basal metabolic rate adjusts to the reduction in kilocalories ingested (by dieting) or the increase in kilocalories burned (by exercise), a balance between caloric intake and burn is once again achieved, and weight loss stops.

Maintaining metabolic homeostasis through a reduction in basal metabolic rate means that there’s only so far you can go when trying to lose weight by dieting, exercise or both.  If you want to keep losing weight through dieting, you have to continue reducing the number of kilocalories you ingest every day.  If you want to continue losing weight through exercise such as riding the bike, you have to keep increasing the intensity of the exercise.

This is why it was recommended in Riding the Bike to Lose Weight that you continually try to ride harder, longer, faster, stronger every time out on the bike if you want to lose weight.  If the intensity of the exercise remains the same, basal metabolic rate will adapt to it and weight loss will stop.  If the intensity keeps increasing, basal metabolic rate will lag behind and weight loss can continue.

If this were the whole story about metabolic homeostasis it would be simple.  If you enjoy riding the bike, figure out ways to put more time and energy into riding the bike and forget about worrying about calories and weight loss.  You will be doing something you enjoy, you will be thinking about something you enjoy and you will most likely lose weight.  The hope is that when you get to the point where you are putting all the time and energy you can or want to into the bike, your weight will have dropped to a level you like.

Unfortunately, it’s not this simple.  Riding the bike (or any other form of exercise) makes you hungry, makes you want to eat more. In addition, men and women are affected differently by this increase in the desire to eat after exercise.  More on this in the next post in the series.