Cycling’s Impact on Life Expectancy

20 Dec

My wife came home the other day shaken up because she was nearly hit by an SUV running a red light on Naito Parkway in downtown Portland. Not surprisingly, it made her question whether it was smart to be riding a bicycle. Ever the rationalist, I said that I was sure that the health benefits gained through the regular exercise of riding a bicycle most likely outweighed the risk of being involved in a crash. (A recent report from OHSU looked at how frequently cyclists are involved in injurious crashes).  There have been studies done on the risks and benefits of cycling (e.g. this one from Cavill and Davis), but I wondered if, on the most basic level – life expectancy – there is data to support an argument one way or another?  If you want to live longer, should you or shouldn’t you ride your bike to work?  Here’s a quick, semi-actuarial look into that question.

Health Benefits of Cycling:

First, there is ample data that regular exercise is important to heart health and that Americans in particular are pretty bad at getting enough exercise.  Studies have demonstrated that a few hours of exercise per week can lead to health outcomes that result in longer life expectancy.  According to a 2005 article in the Washington Post, there is strong evidence that “People who engaged in moderate activity — the equivalent of walking for 30 minutes a day for five days a week — lived about 1.3 to 1.5 years longer than those who were less active. Those who took on more intense exercise — the equivalent of running half an hour a day five days every week — extended their lives by about 3.5 to 3.7 years, the researchers found.”

Another report (Cavill and Davis 2003) discusses a number of studies relating to the potential health benefits and risks of cycling.  One finding is that Bicycling improves fitness more than walking and leads to increased cardiovascular fitness: “Dutch research has demonstrated that cycling as part of normal daily activities can yield much the same improvements in physical performance as specific training programmes. The higher the total distance cycled during a 6 month trial period of activity among men and women who had not participated in regular intensive exercise over the previous six months, the higher the gain in maximal external power and maximal oxygen uptake. For those with a low initial fitness level, a single trip distance of 3 kilometres per day each way was found to be enough to improve physical performance if repeated at least three times a week. This matches other research which has found that men with low cardiorespiratory fitness who became fitter had a lower risk of cardiovascular disease mortality than men who remained at a low level of fitness. Research from Finland provides some of the strongest evidence for the health benefits of cycling. Two studies suggest that journeys to and from work by bicycle provide exercise of sufficient intensity and duration to improve fitness and health, and that travel by bicycle provides greater increases in measured fitness than does walking” (pg 16-17).

In another study cited by Cavill and Davis, they find that cycling helps with a number of health related outcomes and decreases mortality 39%: “The most substantive study in this category is the Copenhagen Heart study, which involved 13, 375 women and 17, 265 men aged between 20- 93 years who were randomly selected from a population of 90,000 living in central Copenhagen. Of this cohort 14,976 cycled regularly and of whom 6,954 cycled to work. The average time spent cycling in those who did cycle to work was three hours per week. The study found that cycling has a strong protective function. Assessed by self-reported health, blood pressure, cholesterol, Body Mass Index, and risk factors such as smoking, the researchers concluded that: ‘even after adjustment for other risk factors, including leisure time physical activity, those who did not cycle to work experienced a 39% higher mortality rate than those who did’ “(pg 17).

Mortality risks of cycling:

The other side of the coin concerns the risks associated with cycling – foremost amongst them, what are the chances that you will be killed while cycling.  Cavill and Davis (see pg 22 here) present some statistics from England showing fatalities per billion kilometers travelled.  Their data show a few interesting things.  Fatalities have been falling for all modes (walking, cycling and driving, between 1981 and 2000).  In 2000, there were 30 fatalities per billion kilometers travelled for cyclists, compared to 48 for pedestrians and 3 for cars.  However, because people drive so many more miles than they cycle, the risk per exposure hour is much higher.

Ken Kifer*’s website has a good discussion of the risks of cycling based on various data sources and analyses.  One source he cites shows that driving (motoring) presents almost twice the risk of cycling (.47 fatalities per exposure hour for driving v. .26 for cycling).  Here’s a table from Kifer’s site showing risks for a variety of activities:

Fatalities per Million Exposure Hours
Skydiving 128.71 Snowmobiling .88
General Flying 15.58 Motoring .47
Motorcycling 8.80 Water skiing .28
Scuba Diving 1.98 Bicycling .26
Living 1.53 Airline Flying .15
Swimming 1.07 Hunting .08
Data compiled by Failure Analysis Associates, Inc.

Let’s take an example:

Working through a pair of  examples.  One is a cyclist (Cyclist A) who bikes 5 miles each work day (2.5 miles each way, or about ~15 minutes each way to and from work).  This equates to half an hour of moderate to rigorous exercise 5 times a week.  By my calculation this puts them somewhat above the category of walking half an hour a day, which corresponds to an increase in life expectancy of 1.3 to 1.5 years.  To be conservative, we’ll call this 1.5 years of increased life expectancy.

Another (Cyclist B) bicycles twice as far – 10 miles each work day (5 miles each way, or ~30 minutes each way to and from work).  They would fall into the category of active for an hour (of at least moderate to rigorous).  This puts them considerably above the category of walking half an hour a day (remember that corresponded to an increase in life expectancy of 1.3 to 1.5 years) and closer to, if not above, the category of intense exercise for 5 days a week (and the 3.5 to 3.7 years of added life expectancy).  We’ll call this 3.7 years of added life expectancy.

Using the risk of fatality by exposure calculations discussed above, the cyclist who bikes 2.5 miles to work each day would have about a 1 to 1.15 in 1000 chance of being in a fatal bicycle accident over the course of 30 years of bicycling, whereas the cyclist who bikes 5 miles would have a 2 to 2.3 in 1000 chance.  Next, I assume that any fatality is equally likely to occur over the course of 30 years, and that a fatality in year 1 equates to a loss of 50 years of life (year 2 = 49 years loss, etc).  Using these inputs, the first cyclist can expect to lose .035 to .04 years to this risk, while the second can expect to lose .069 to .08 years (here’s a copy of the excel worksheet I used to come up with these numbers).  To sum up:

  • Cyclist A (5 miles a day): Gains an expected 1.5 years through increased health and loses an expected .04 years to the risk of a fatal accident.  This is a net gain of 1.46 years of life expectancy.
  • Cyclist B (10 miles a day): Gains an expected 3.7 years through increased health and loses an expected .08 years to the risk of a fatal accident.  A net gain of 3.62 years of life expectancy.

While the data used in this quick analysis merit much more research and scrutiny, the fact is that, when considered on a purely rational basis, the positive health benefits of cycling regularly far outweigh the risks. In fact, even if the health gains were considerably less than assumed here (say half as great) and the risks were considerably greater than assumed here (say twice as high), the benefits still outweigh the risk by a long shot.  While the fear of being hit by a car is a really great reason for cyclists to be absolutely defensive “drivers,” it is in fact counterproductive in terms of prolonging your life if you allow it to prevent you from choosing to cycle.

I should note that I have had at least two near misses with cars running red lights in the past year – both times I felt I came near to being knocked off.  One of those instances was while riding a bike, a car blew a red light and slammed on their breaks at the last minute skidding to a stop in the middle of the intersection, about 10 feet from me.  The other time, though, was when I was in a car.  I was pulling out onto a busy and fast-moving arterial.  I had a green light and a car sped through a red light at about 40 mph right in front of me.  Had I reacted a little faster to the green light, I would have been right in his path.  Part of the lesson here is that, people in cars die in accidents too – and in reality the choice is often between bicycling and driving, not between bicycling and locking oneself up in a room full of pillows – thus the gains really outweigh the risks to a greater degree than this analysis projects.

* It is somewhat of a bitter irony to this post that Ken Kifer, an ardent bike advocate, died when a drunk driver hit him on his bicycle.  I think, though, that it’s important to note that the culprit was the drunk driver, not that bicycle, and that many more people are killed by drunk drivers (over 13,000 in 2008) while driving than while bicycling.

One Response to “Cycling’s Impact on Life Expectancy”

  1. Nathan January 29, 2011 at 3:15 am #

    Here’s another article I came across comparing the health benefits and risks of cycling:

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