By spreading out and riding further apart as a motorist attempts to overtake, cyclists riding in groups can reduce their exposure to harmful vehicle emissions, a recent study has found.
That’s because, according to academics specialising in aerodynamics and environmental flow, pollutants from the exhausts of passing cars can become trapped within a tightly-packed group of cyclists, thanks to a “complex aerodynamic field” generated by the riders.
The study, carried out by researchers at the University of Surrey, assessed the different ways in which cyclists can be exposed to car fumes while riding in groups in urban environments through a series of experiments conducted at the university’s Environmental Flow (EnFlo) wind tunnel.
The researchers, whose findings have been published in the Journal of Wind Engineering and Industrial Aerodynamics, captured results – using a scale model of a typical London street – from a vehicle positioned in front of the four riders and one adjacent to them, replicating an overtaking position.
They measured the pollutant concentration around the cyclists based on the position of both the car exhaust and the riders, as well as the wind direction and strength.
The findings confirmed that, when a motorist is driving in front of a group and there is little wind, the pollutant exposure decreases the further a cyclist is from the vehicle, regardless of their position within the group.
When the wind is stronger, however, positioning with the group becomes more important, and the researchers say that riding towards the back on a blustery day can be a good strategy to minimise exposure.
But when a driver is passing a group of cyclists, things become more complicated. According to the results of the experiment, when a vehicle is adjacent to a group of cyclists, the exhaust fumes can be trapped by what the researchers describe as a “complex aerodynamic field” generated by the riders (similar to the effects of drafting), increasing their exposure to harmful pollutants.
In the case of a passing driver, then, riding at the front of the group – regardless of your proximity to the car exhaust – will minimise your exposure to fumes.
To mitigate the effects of pollution on all members of the group the researchers recommend that, when a motorist is overtaking, the riders within it should spread out, allowing some of the trapped fumes to escape.
Overtaking drivers are also recommended to give cyclists as much space as possible when passing, and to avoid pulling too sharply in front of them, thereby increasing the proximity of the fumes emanating from their exhaust.
“Cycling is encouraged to reduce congestion on the roads, as well as traffic emissions, yet despite many encouraging health aspects of cycling, the exposure to and inhalation of vehicle pollutants is something not to be forgotten, especially when used as a regular alternative transport method,” says Joy Schmeer, a postgraduate researcher at the University of Surrey and the lead author of the paper.
“The findings of these experiments highlight group cyclists needs to consider their routes and position within a group, especially when roads become busier and narrower.”
The new research, as well as advising pollution-conscious cyclists on the best riding position within a group, also suggests that group riding should be considered by urban planners when designing mitigation strategies to minimise cyclists’ exposure to fumes within towns and cities, especially where busy and narrow cycle lanes often result in cyclists riding in line.
According to the study’s conclusion, as being as far away as possible from both polluting vehicles as well as other cyclists is key to avoiding exposure, policy makers should therefore be advised to construct wider cycle paths or, “even better” (in the words of the researchers), completely separate riders from road traffic.
“The results of these experiments reveal important recommendations that cyclists and drivers should know to increase health and safety while cycling in groups,” Dr Marco Placidi, Senior Lecturer in Experimental Fluid Mechanics at the University of Surrey, added.
“While drivers need to maximise their distance away from riders before overtaking them, cyclists should aim to distance themselves from the vehicle’s exhaust, but also potentially from other riders if a vehicle is driving adjacent to them.
“As for further recommendations, experiments like these show the need to consider repercussion of peak utilisation of urban cycle lanes during their design stages.”
Ryan joined road.cc as a news writer in December 2021. He has written about cycling and some ball-centric sports for various websites, newspapers, magazines and radio. Before returning to writing about cycling full-time, he completed a PhD in History and published a book and numerous academic articles on religion and politics in Victorian Britain and Ireland (though he remained committed to boring his university colleagues and students with endless cycling trivia). He can be found riding his bike very slowly through the Dromara Hills of Co. Down.