Friday February 14 2014
There are around 10,000 "Boris bikes" in London
"London's cycle hire scheme has had a positive effect on the health of its users," BBC News reports. So-called "Boris bikes" (named for the "colourful" Mayor of London, Boris Johnson) have led to improvements in both physical and mental health, researchers report.
The headlines follow the publication of a study in the BMJ that aimed to model the health impacts of the London cycle hire scheme. The scheme was introduced in 2010 and is now reported to comprise around 10,000 bikes.
The study looked at usage data on the bike scheme and modelled estimated levels of physical activity against the reduction in chronic diseases this would be expected to give for both physical and mental health conditions.
Researchers took the potential harms of cycling into account, including involvement in traffic accidents and exposure to pollution.
They found that even after traffic injury rates were accounted for, benefits were still seen in terms of the health problems avoided. And the rate of accidents was actually lower than expected.
The estimated benefit was greater for men than it was for women, and was greater for older age groups than younger groups. However, it should be noted that the majority of cyclists were men of working age – there was less data for women and older people, so the estimated health benefits for these groups may be less accurate.
Despite the belief that cycling on the streets of London exposes you to pollution, pollution levels were actually higher on the London Underground, which people may have otherwise used to travel.
It is important to bear in mind that the expected health benefits discussed in this study are only estimates. Nevertheless, this study supports current recommendations for leading an active, healthy life, such as cycling.
Where did the story come from?
The study was carried out by researchers from the University of Cambridge, University College London and the London School of Hygiene and Tropical Medicine, and was funded by the British Heart Foundation, the Economic and Social Research Council, the Medical Research Council, the National Institute for Health Research and the Wellcome Trust.
It was published in the peer-reviewed British Medical Journal. The study has been made available on an open access basis, so it is free to read online.
The UK media's reporting of this story was generally accurate, although The Guardian's headline "Boris bikes benefit older cyclists more" must be interpreted in the correct context. This effect is likely to be the result of very few cycle trips being made by older adults and very few recorded cycle injuries involving older adults.
A final point is that the widely used term "Boris bikes" seems a little unfair to the former Mayor of London, Ken Livingstone, who actually initiated the scheme, although admittedly "Ken's bikes" doesn't have the same ring to it.
What kind of research was this?
This was a modelling study that aimed to estimate the health impacts of the bicycle hire system in London.
Regular physical activity is widely known to improve health and wellbeing and reduce the risk of many chronic diseases. What is termed "active mobility" has been suggested as a key feature of a "healthy city", and is expected to yield health, economic and environmental benefits.
Bicycle hire systems are one of the ways that cities can try to encourage this. These schemes involve making bicycles available to hire from self-service bike stations and can be dropped off at any other station across the city, turning cycling into a form of easily accessible public transport.
The London cycle hire scheme was launched in July 2010. After a south-west extension in December 2013, the scheme now includes around 10,000 bicycles and 723 docking stations. Users can either register online for an access key (registered users) or pay by card at a docking station (casual users).
The authors of this study modelled the impact of London's cycle sharing system on the health of its users through access to complete registration and usage data.
The researchers specifically looked at the effects on mortality and morbidity (illness) by gender and age, as well as estimating the detrimental effects of road traffic injury and exposure to air pollution. Essentially, they wanted to see if the health benefits of cycling through regular exercise outweighed the potential risk of accidental injury and air pollution.
The main difficulty with this type of study is that, while using the best available data, it can only produce estimates and may have varying degrees of accuracy.
What did the research involve?
The researchers modelled the health impacts of the London cycle hire scheme by comparing its effects against a situation where it didn't exist. They were mainly interested in changes in disability adjusted life years (DALYs). These are calculated by adding up the years of life lost because of premature death, and years of healthy life lost because of disability. The researchers looked at yearly changes in users' incidence of disease and injury to calculate the impacts on their life course.
Transport for London (TfL) provided data on all cycle hire trips made since the scheme started up to March 2012 so the researchers could examine its usage. For the final 12 months, TfL also provided trip level data, including a unique ID for each user, giving the start time, end time and location of each trip made.
They also had information on registered users' gender and where they lived. No personal data was available for casual users. Two online surveys were conducted by TfL in 2011, which gave age and sex information on 2,652 registered and 1,034 casual users, as well as what transport they would use if they weren't hiring the bikes.
Physical activity was modelled for men and women and by age using metabolic equivalent of tasks (MET), a measuring system based on how much energy the metabolism "burns off" during a specific physical activity. It was assumed that:
- the baseline activity levels of cycle hire users were somewhere between that of existing cyclists and that of the general population, and allowed for the possibility that cycle hire may appeal more to those who are already more physically active
- use of the cycle hire scheme bicycles increased by the amount observed in the operational data, while the user's own bicycle use and walking decreased by the estimated duration of the cycle hire trips
- other activities (for example, sport) remained unchanged
Using previous research, the authors incorporated existing estimates of how much physical activity reduces the risk of developing various chronic diseases, such as cardiovascular disease and cancer, and the relative risk of dying from any cause.
The researchers also took the potential harms from road traffic injuries into account. This was estimated using two different methods:
- using data routinely collected by London's Metropolitan Police Service and TfL, they looked at the number of recorded injuries where a cycle hire bike was involved against the total travel time using the hire bikes recorded by operational usage data
- using police data on the number of cyclists involved in accidents in the cycle hire zone, they assumed that the rate of injury on hired cycles was the same as the rate of all cycling incidents in central London between 2005 and 2011, compared with the total travel time in central London
The researchers also estimated the effect of air pollution PM2.5 (particles with a diameter of ≤2.5 micrometres), which was calculated by looking at the exposure rate along the most likely route for each cycle trip, taking factors such as measured pollution, road position and ventilation rates into account.
What were the basic results?
Between April 2011 and March 2012, 578,607 unique cycle hire users made a total of 7.4 million cycle hire trips with 2.1 million hours of use. This makes up 12% of the estimated 61.2 million cycle trips made that year in the cycle hire zone, and 10% of the estimated 20.8 million hours of cycling duration.
Around three-quarters of these 7.4 million cycle hire trips were made by men (71%) and by people in the 15 to 44 age bracket (78%). Fewer than 2.5% of cycle trips were made by those over the age of 60. Compared with the figures for cycling in London in general, there were fewer fatalities and injuries on cycle hire bicycles than would be expected.
After taking all injuries involving the hire bikes into account, this study found an overall positive health benefit from use of the bike hire scheme:
- use of hire cycles by men gave a reduction in 72 DALYs for the whole male population of cycle users (95% credible interval 43 to 110 reduction) – a reduction in the number of years of life living with disability is a positive benefit
- for women, use of hire cycles gave a slightly smaller benefit of a reduction of 15 DALYs (95% credible interval 6 to 42 reduction)
Most of the reduction in DALYs in men was related to estimated reductions in heart disease, while in women it was a reduction in depression.
When the rate of cycle hire injury was assumed to be equivalent to the background rates of all cycling injuries in central London, the benefits were still positive for men but they were smaller (only a reduction of 49 DALYs, 95% confidence interval [CI] 17 to 88 reduction).
However, the benefit was no longer significant for women. As the researchers say, this sex difference largely reflects the fact that observed road collisions involving women have had a higher fatality rate.
Because of the higher observed injury rates among younger people (15 to 29 years), the benefits in terms of DALYs were greater for older people, even though they didn't use the bikes as much.
Looking at air pollution, PM2.5 was lower on all routes by road than the reported levels on the London Underground at the time. The benefit from the avoided exposure to PM2.5 on the Underground balanced out the harms from increased inhalation of pollutants because of the higher ventilation rates associated with cycling.
How did the researchers interpret the results?
The researchers conclude that, "London's bicycle sharing system has positive health impacts overall, but these benefits are clearer for men than for women and for older users than for younger users. The potential benefits of cycling may not currently apply to all groups in all settings."
This modelling study estimated that, on the whole, the health benefits of the bike hire scheme in London outweighed the harms from cycle-related injuries and deaths, as well as pollution. The benefits were smaller for women than men, as well as for younger groups, suggesting that the risk reward on the roads might not be same for everyone.
The researchers found that around three-quarters of cycle hire trips were taken by men, and mostly in the age group of 15 to 44 years. The estimated benefit was greater for men than for women, although this may be because men used the bikes more than women in this period. Despite this, there has been a higher rate of cycling fatalities involving women, particularly in incidents with large goods vehicles (LGVs).
Similarly, the benefit was observed to be greater for older people, although this should be interpreted in the correct context. This was because of the higher rate of injuries involving younger people and only 2.5% of all cycle hire trips were made by people over 60.
Despite the possible belief that cycling in London exposes you to pollution, pollution levels were actually higher on the London Underground, which people may have travelled on if they weren't cycling.
The study's strengths include its use of reliable bike usage data and that it took recorded cycle accident rates and recorded pollution levels into account.
However, it is important to bear in mind that the expected health benefits were only estimates. That said, the estimates of bike usage and accidents are likely to be fairly reliable. But there are other areas where the estimates may not be so accurate, particularly the researchers' estimates of cyclists' baseline physical activity levels and their general activity levels. The researchers had no information on this, so this is one area where there could be considerable inaccuracy.
The expected reduction in the risk of chronic diseases and mortality with increased physical activity are only estimates from previous research studies. Researchers also may not have been able to capture all possible health effects. Similarly, using generalised pollution estimates may not be a reliable way of assessing individual exposure to pollution, which could vary around the average considerably, influencing the risk reward profile of each cyclist.
Despite these limitations, the results of this study suggest that the benefits of cycling strongly outweigh the potential risks. Cycling arguably offers a combination of three significant benefits: it is far cheaper than running a car, it is convenient, with no hanging around at bus stops or tube stations, and it helps keep you healthy. Read more about the benefits of cycling.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.