The researchers study the impact of e-bikes in Portland, Oregon, and note that there are other benefits besides CO2 emission reductions, such as being "rewarding and fun for many users, is freeing for users with limited ability and mobility, and can even lead to a car-free household." They also confirm that people ride farther on e-bikes than they do on regular bikes, "meaning that e-bikes offer the opportunity to multiply the benefits already available through conventional cycling."
The researchers used data from a previous study of e-bike usage that examined e-bike users and how their transportation changed from driving or transit. Then they did the pretty straightforward mathematics for Personal Miles Traveled (PMT) by each form of transportation times the emissions for each with "local information about population, trip generation by mode, trip length by mode, auto occupancy, auto fuel economy, transit fuel economy by person mile, and e-bike emissions rate as inputs." They used Portland, Oregon as the case study because "of the availability of regional transportation data and the extensiveness of the city’s bike network that lends itself to e-bike uptake."
The study took into account the emissions profile of the local power supply but really, the energy used by an e-bike is so small that it barely matters; even with the dirtiest coal-fired power you get average emissions 12.568 grams/mile whereas Portland is 4.9 grams/mile; a car is 274.
In this case, we found that a 15% e-bike mode share by PMT could result in a 12% reduction in transportation CO2 emissions, with an average CO2 savings of 225 kg per e-bike per year. We selected the 15% mode share arbitrarily as the optimistic maximum of a range of potential e-bike regional mode share values.
While a 12% reduction in transport emissions is great, we need more. We need a much greater mode share than 15% to really make a difference. E-bikes make that easier; the authors note that "e-bikes have been shown to reduce barriers for riders with disabilities and mobility issues, older riders, and female riders compared to conventional bicycles."
They also suggest other measures to increase ridership, including subsidies, charging, and parking facilities, noting that "Reducing the speeds and volumes of motor vehicles and building separated bike lanes or “superhighways” could also help to increase e-cycling." Being Portland, perhaps they could also sell good quality discount rain gear. The authors conclude:
E-bikes offer regions a solution to accelerate the uptake of cycling as an alternative to the automobile for taking utilitarian trips. By making e-bikes an integral part of the local mode share, regions can substantially decrease greenhouse gas emissions and automotive PMT. As was estimated in Portland, OR, this could be a reduction on the order of 1,000 metric tons CO2 per day or 225 kg CO2 per e-bike per year, on average, at the 15% PMT mode share case. Significant political will and effort may be required, however, to seize this opportunity. The model presented here is useful for helping regional stakeholders see this potential so that an informed decision can be made to include e-bike promotion as part of a larger suite of carbon emission reduction initiatives.
An earlier version of the study listed lifecycle emissions for various modes of transport, finding that "bicycles and e-bikes have a lifecycle emissions rate of approximately 21 grams and 22 grams of CO2e per person kilometer respectively, while public transit buses emit 101 g lifecycle CO2e and cars emit 271 g lifecycle CO2e per person kilometer." Bikes and e-bikes have less than a tenth of the carbon footprint of cars, and that's not even counting the carbon in the parking garages and any new roads.
Fifteen percent is not nearly ambitious enough; imagine the difference if it were double that.