Group riding: Cyclists exposure to road vehicle emissions in urban environments

Abstract

A series of wind tunnel experiments were conducted in the University of Surrey’s Environmental Flow wind tunnel with a 1:50 scale of a typical London street canyon to assess the exposure of cyclists riding in a group to the emissions of polluting vehicles. A propane source emitted from an Ahmed body was used to model a car exhaust and a fast flame ionisation detector was used to measure pollutant concentration around four cyclists for multiple configurations of the source, cyclists, and wind directions. Two cases were investigated with a vehicle driving in front of a line of cyclists and adjacent to them (as if it were overtaking them).In the first case, for small wind incidence, findings confirm that the cyclists exposure decreases exponentially with their distance from the source with a small dependence on wind direction but largely independently of the riders position within the group. For large wind incidences, typical of urban canyons, the rider position within the group becomes more important.For the second set of experiments, with the vehicle positioned adjacent to the riders, it was found to be preferable for a rider to be in front of the group regardless of the distance from the source, as this results in lower exposure to pollutants. This is likely linked with the complex aerodynamic field generated by the group of riders that can trap the vehicle exhaust fumes amongst the cyclists, hence increasing the exposure.This research suggests that group riding should be considered when designing mitigation strategies to minimise cyclists exposure to road traffic pollution within urban environments, where busy and narrow cycle lanes often results in cyclists riding in line.