It takes about 18 of Vancouver's shared bikes to match the exhaust cut from removing one car

Bikes per car removed is a unit a city can plan with. The authors defined it narrowly: the yearly cut in exhaust emissions, carbon dioxide or toxic exhaust particles, that comes from taking one fossil-fuel car off the road ^[8]. The 18-bike figure assumes today's level of use ^[8].

The gap between that figure and the high-use five is a factor of about 3.6 ^[1]. The difference is utilization. In the high-use case each bike replaces more car trips, so fewer bikes are needed per car's worth of savings ^[9]. The authors wrote that "the environmental value of bike sharing depends not only on how many bikes are available, but on how often they are used to replace trips that would otherwise have been made by car" ^[10].

Those car trips fall within a narrow range of distances. Short commutes in the study averaged about 2 km and leisure rides about 10 km, with e-bike trips slightly longer ^[6]. Car-driver trips currently outnumber bike trips about four to one ^[2]. The 11% figure for bikeable trips comes from a City of Vancouver survey, while the mode shares come from the authors' TransLink analysis ^[7][5][4]. The gap between 11% and 7% is therefore not a clean count of trips still available to win ^[5][7].

The study does not report how many shared-bike rides actually replaced a car. The authors focused on trips "with the potential to replace car journeys" ^[15]. A ride that would otherwise have been a walk or a bus trip removes no car exhaust, so both the 18 and the five rest on substitution assumptions inside the models ^[4]. The published summary does not give the rides per bike per day behind either scenario, or say whether rebalancing vans and bike manufacture are counted.

The study's numbers are emissions. The congestion case comes from the authors' discussion of electric cars. Canada's federal government aims for 75% of cars sold to be electric by 2035 ^[11]. Electric cars occupy the same road space as the cars they replace, so electrification alone does little for traffic, the authors argue ^[12]. EVs are heavier, battery production can be carbon-intensive, some provinces still generate power partly from fossil fuels, and tire, brake and road wear keep producing particles ^[13]. The authors also expect that Canadians with EVs will likely drive more ^[16]. They still call electrification "an essential part of decarbonizing transportation" ^[14].

Transport is almost a quarter of Canada's greenhouse gas emissions and about 40% of Metro Vancouver's ^[1]. In 2015, transport-related air pollution was associated with more than 1,200 premature deaths in Canada ^[2]. I think the bike-share case holds under the conditions the authors set: short trips in dense areas ^[3], bikes that are ridden often ^[10], and rides that would otherwise have been drives ^[15]. On that evidence, bike share belongs beside electrification in a city plan, for trips of a few kilometres ^[6][14].