This article was written for CAN's Chainlinks magazine, but we are only printing an excerpt of it in Chanlinks, due to constraints on space.
Authors: Alistair Woodward, Graeme Lindsay, Alex Macmillan
School of Population Health in the University of Auckland
The risks of cycling are mostly obvious. Many riders have suffered injuries as a result of a crash, or know of people who have been injured. In a study of those who took part in the 2006 Taupo Cycle Challenge, one-third told us they had suffered a significant crash in the last 12 months, and 13% reported an injury in the same time period that was serious enough for them to go and see a doctor. There are other hazards on the road, perhaps not so apparent, but important all the same. Cycling in heavy traffic means breathing in the fumes from exhausts of cars, motorbikes, buses and trucks. The emissions include carbon monoxide, fine particles and other noxious substances. And because cyclists are working harder than car drivers and passengers, and breathing more rapidly and more deeply, more pollution can get into the body than with other road users.
Given all the dangers they encounter, you might expect that people who ride frequently have poorer health than the rest of the population. But this isn't the case. A famous study in Denmark followed up 30,000 adults over 14 years. About a quarter cycled to work, and mortality in this group was 30% less than the rest of the study population. Did those who were relatively fit choose to cycle to work, and those who were less healthy travelled by other means? Maybe in some cases, but it was a healthy population at the beginning of the study, and on the whole the cyclists and non-cyclists were very similar in terms of their smoking habits, body weight and other risks for heart disease. And the investigators also took into account that those who cycled to work might tend to be more active out of work hours. Differences in leisure time physical activity didn't explain the much better health of the cycle commuters.
The Danish study was not an experiment (not so easy to do in free-living human populations!), and the difference that was seen probably overstates the size of the effect that would be achieved if everyone took up riding a bike to work. But there is good reason to believe that getting more people to take active transport more often would indeed be health-promoting. We know that physical activity is one of the cheapest, safest and most effective treatments. Even small amounts of exercise, taken regularly, lower risks of heart disease, cancer and diabetes, and improve many measures of mental health. And there is a cleardirect relation between the amount of exercise that is undertaken and the health benefits that follow (up to a limit of course), so vigorous activity is part of the preventive medicine agenda too. In other words, it is entirely plausible that the extra physical demands of cycling to work (3 hours a week, on average, in the Danish study) would make a difference to subsequent health, and take note: this exercise benefit more than cancelled out effects of road crash injury or illness caused by air pollution.
But isn't Denmark a rather special case? The whole country, and Copenhagen in particular, is famous now for being bike-friendly, and Danish streets are certainly safer and less polluted than most other places. But the study of commuting and health started in the 1960s, when cycling infrastructure in Denmark was relatively primitive. What is more, studies from other parts of the world have produced similar results. A large group of women in Shanghai in the early 2000s were followed up in the same way, and again, those who cycled for transport had lower mortality rates than their peers. At this time Shanghai was moving rapidly to a car-oriented society, with few concessions for bicyclists, and high levels of pollution.
After learning about the research that had been done overseas, we asked ourselves the question, what would happen in New Zealand if we were able to convert a small proportion of short car trips in the city to bicycle trips? What would be the balance of health gains and losses? We have tried to estimate also the environmental effects.
We included urban centres (10,000 inhabitants or more), and travel for all purposes except work business. Short trips were defined as 7 km or less and we looked at the effect of a 5% shift from cars to bikes. (Note that this would not be particularly radical - such a move would simply return the number of bikes on the road in New Zealand to the levels seen in the 1980s.)
Each year, this scenario would cut vehicle travel by approximately 223 million kilometres and save about 22 million litres of fuel and around $40 million in fuel bills. We estimated the savings in greenhouse emissions would amount to about 50,000 tonnes of CO2 per year. What would be the gains and losses health-wise? We projected that this increase in cycling would lead to an additional five cyclist fatalities a year from crashes with motor vehicles. However, these additional fatalities would be balanced by about six fewer deaths caused by air pollution from vehicles, and there also would be about 116 fewer deaths each year, due to increased physical activity. We estimated a dollar value of these health effects and found that the net health effect of shifting 5% of short trips from cars to bikes would be a saving of about $193 million per year.
Others have made calculations like this, in Europe and North America, and the results are similar. But there are many approximations and assumptions, and we are working at present to improve the projections.
Improvements in the vehicle fleet are likely to significantly reduce air pollution levels at ground level , although this may be counter-acted by increasing traffic levels. We assumed in our calculations that the injury risk for cyclists depends on the numbers of cyclists on the road, and that as cycling becomes more common, it become safer (per kilometre ridden). But there may be thresholds, and we don't know exactly how common cycling needs to be for the "safety in numbers" effect to kick in. We have assumed that there are no changes in the road environment. What if the move to provide cycle lanes (and, even more adventurously, cycle paths) was accelerated? We are also examining more closely the health benefits of physical activity, including different effects by age, gender and ethnicity, with an attempt to take account of the timing of the effects (Put more people on the road and injury risks apply immediately; however, many of the health benefits may take a while to act.)
It is difficult to be precise about the numbers, however, the take home message is robust - cycling has pluses and minuses, but overall, the health benefits of shifting from four wheels to two outweigh the risks.
Note: this material was also made into a presentation called "put me Back on my Bike" which is available online here as either a podcast or a pdf file.