Curitiba, Brazil, developed the first bus rapid transit (BRT) system in the 1970s, a model replicated in more than 200 cities worldwide. Dedicated BRT lanes along main thoroughfares—separate corridors from automobiles—were installed for fifty times less than the cost of rail. Bus stops were designed to be more like metro stations with multiple points of entry and exit.
All mass transit modes use scale to their emissions advantage. When someone opts to ride a streetcar, bus, or subway rather than driving a car or hailing a cab, greenhouse gases are averted.
The benefits go beyond emissions reduction and accrue to all city dwellers, not just those who use mass transit. By reducing the volume of cars, mass transit relieves traffic congestion. With fewer people driving, fewer accidents and fatalities take place. Overall, air pollution drops. Mass transit also makes cities more equitable by providing mobility to those who cannot drive.
Urban transport is the single largest source of transportation-related emissions, and growing. With good urban design, mass transit can help embed mobility, livability, and sustainability in cities.
Curitiba, Brazil…Jaime Lerner: Adler, David. “Story of Cities #37: How Radical Ideas Turned Curitiba into Brazil’s ‘Green Capital.’” The Guardian, May 6, 2016.
dedicated lanes…installation costs [vs.] rail: Reed, Drew. “How Curitiba’s BRT Stations Sparked a Transport Revolution.” The Guardian. May 26, 2015.
Curitiba’s…passengers [vs.] London’s Tube: Adler, “Green Capital.”
bus rapid transit…replicated…worldwide: Global BRT Data. http://brtdata.org/.
transport sector…emissions: Sims R., R. Schaeffer, F. Creutzig, X. Cruz-Núñez, M. D’Agosto, D. Dimitriu, M.J. Figueroa Meza, L. Fulton, S. Kobayashi, O. Lah, A. McKinnon, P. Newman, M. Ouyang, J.J. Schauer, D. Sperling, and G. Tiwari. “Transport.” In Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK, and New York: Cambridge University Press, 2014.
Urban transport…use of cars: Pyper, Julia. “Cars Will Cook the Planet Absent Shift to Public Transportation.” Scientific American. September 17, 2014.
United States…daily commuters: Cox, Wendell. “Evaluating Urban Rail.” NewGeography.com. December 5, 2014.
[vs.] Singapore and London: UN-HABITAT. Planning and Design for Sustainable Urban Mobility: Global Report on Human Settlements 2013. United Nations Human Settlements Programme, 2013.
fewer accidents and fatalities: Pourbaix, J. “Towards a Smart Future for Cities: Urban Transport Scenarios for 2025.” Public Transport International, 60, no. 3 (2011): 8-10.
transit…makes cities more equitable: Zhao, X., A. Mahendra, N. Godfrey,,H. Dalkmann, P. Rode, and G. Floater. Unlocking the Power of Urban Transit Systems for Better Growth and a Better Climate. London and Washington, D.C.: New Climate Economy, 2015.
“A train is a small society”: Gopnik, Adam. “The Plot Against Trains.” The New Yorker. May 15, 2015.
Project Drawdown defines the mass transit solution as: the increased usage of mass transit or public transport to get around cities. This solution replaces the use of conventional internal combustion engine (ICE) cars.
Mass transit is a collection of different urban passenger transport modes (e.g. metro, bus, tram), which can facilitate the mobility of large numbers of people. It has long been a structural part of urban environments worldwide, and while its market share has decreased through the second half of the 20th century, it remains the only viable alternative to travel by private car on many occasions. As the population of urban areas is expected to grow from 2020-2050, mass transit can claim an important share of the new travel demand. The increased usage of public transportation reduces the need to drive cars as a form of urban mobility.
Using information from studies and reports conducted by major international institutions, several possible outcomes of various aggressive adoption scenarios were examined.
Total Addressable Market 
The total addressable market for mass transit is defined as the total urban mobility, in passenger-kilometers, projected by institutions such as the International Energy Agency (IEA), the International Council on Clean Transportation (ICCT), and collaborative work between the Institute for Transportation and Development Policy and the University of California–Davis (ITDP/UCD). Current global adoption  of mass transit, excluding non-motorized trips, is 34 percent of the market, boosted mainly by high adoption rates in low-income countries where car ownership is lower. In high-income areas like Europe and Japan, mass transit already has significant and rising adoption; still, conventional ICE vehicles own the largest part of the market share in Europe and North America, as well as in other developed regions. Meanwhile, the adoption of mass transit in Asia and Africa is threatened by a combination of economic growth and poor transit infrastructure, partly leading to a business-as-usual projection of 15 percent global mass transit adoption in 2050 as urban mobility expands rapidly through the use of other modes especially private cars.
Adoption Scenarios 
Impacts of increased adoption of mass transit from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s usage remains fixed at current levels.  Future growth projections of mass transit are aligned with the IEA and ITDP/UCD.
- Plausible Scenario: Estimates of future growth were based on projections of mass transit adoption from the 2016 IEA 2°C Scenario, where adoption reaches 23 percent in 2050.
- Drawdown Scenario: Projections were aligned with the ITDP/UCD 2014 Global High Shift Scenario, where adoption reaches 36 percent in 2050.
- Optimum Scenario: A linear projection from the current adoption to 45 percent adoption by 2050 was used, assuming that 45 percent is the maximum one could expect for global urban mass transit usage.
Emissions calculations capture electricity, fuel use, and indirect emissions for mass transit, all weighted according to the mass transit mode. Electricity and fuel data were obtained from ITDP/UCD and other sources. Car emissions are based on fuel use alone. Fuel economy data is taken from multiple sources and represents the global car fleet.
The financial analysis is conducted from the perspective of the user, rather than mass transit operators, municipalities, or society as a whole. This is valuable so that those making decisions about whether to travel by mass transit or by conventional vehicles have a better idea of the financial and emissions aspects of that decision. Mass transit first costs have been excluded from this model to prevent misinterpretation of the cost of implementation, but car first costs have been included as a fixed operating cost of depreciation. By contrasting the use of mass transit with internal combustion engine vehicles, the depreciated costs of owning and using a car are compared to the cost of purchasing a mass transit ticket. The financial evaluation is thus based on the net operating costs: the weighted average ticket price of mass transit options by region,  compared to the fixed operating costs (that is insurance and depreciation), fuel costs, and other variable operating costs (e.g. maintenance) of car ownership.
To ensure that this model was well integrated with other Drawdown models, we used the common market and car inputs across all relevant solutions. Adoption was also restricted so as to not exceed the total mobility demand represented by the total addressable market.
The Plausible Scenario results in an emissions reduction of 6.6 gigatons of carbon dioxide-equivalent greenhouse gases and over US$2 trillion  in net operations savings, with adoption staying at 23 percent in 2050. In the Drawdown Scenario, emission reductions could reach as high as 17 gigatons, and 26 gigatons in the Optimum Scenario over 2020-2050.
When combined with other technologies and approaches such as electric/hybrid engines, clean energy production, vehicle automation, and car sharing, the impact of mass transit can increase significantly. Without mass transit-focused investments and policy, it is likely that much of the projected growth in urban mobility would be taken up by private cars, and even if exciting developments like autonomous shared-car fleets become widespread, mass transit would be needed on high-demand routes. There therefore needs to be a significant effort to ensure that mass transit systems are attractive means of mobility for the increasingly urbanized world. We excluded mass transit investment costs for governments, but we also excluded additional benefits of increased mass transit usage such as reduced congestion from private vehicles, and potentially more city space reclaimed from parking lots that are not required. It is unlikely that adding these elements would alter the basic conclusion that mass transit is a key part of reducing urban transport emissions.
 For more on the Total Addressable Market for the Transport Sector, click the Sector Summary: Transport link below.
 Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
 For more on Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Transport Sector-specific scenarios, click the Sector Summary: Transport link.
 This assumption is contrary to the standard Project Drawdown Reference Scenario, which assumes that growth occurs in relation to the overall growth of the market, with the percent adoption remaining fixed at the base year levels. Given current trends, this assumption is considered to be too optimistic without strong policy in countries such as China and India where growth in conventional car ownership is on the rise.
 Ticket data came from http://www.worldatlas.com/articles/cost-of-public-transportation-around-the-world.html, and has been averaged by Drawdown Region.
 All monetary values are presented in US2014$.
Full models and technical reports coming in late 2017.