In 2015, the Oxford English Dictionary added ride-share to its pages. Ridesharing is the act of filling empty seats by pairing drivers and riders who share common origins, destinations, or stops en route. When trips are pooled, people split costs, ease traffic, and lighten the load on infrastructure, while curtailing emissions per person.
A wave of technologies has accelerated ridesharing’s popularity:
- Smartphones allow people to share real-time information about where they are and where they are going.
- The algorithms that match them with others and map the best routes are improving daily.
- Social networks are buoying trust, so individuals are more likely to hop in with someone they have not met.
Getting people to double or triple up in their cars is not always easy. When fuel is cheap, carpooling declines. An abundance of free or cheap parking also steers people to journey solo. So does the desire for autonomy, privacy, and expedience.
For many, cars have seemed indispensable to day-to-day life. But, increasingly, mobility is seen as a service to access. When cars are used collaboratively, you can catch a glimpse of the future—one with fewer cars overall.
Oxford English Dictionary…ride-share: Steinmetz, Katy. “Oxford Dictionaries Adds Janky, EGOT and Ridesharing.” TIME. February 26, 2015.
“ride alone…ride with Hitler!”: Pursell, Weimer. When You Ride Alone You Ride with Hitler! Government Printing Office for the Office of Price Administration, NARA Still Picture Branch, 1943. https://www.archives.gov/exhibits/powers_of_persuasion/use_it_up/images_html/ride_with_hitler.html.
1970s, the heyday of ridesharing: Chan, Nelson D., and Susan A. Shaheen. “Ridesharing in North America: Past, Present, and Future.” Transport Reviews, 32, no. 1 (2012): 93-112.
2008…10 percent of Americans commuted jointly: Chan and Shaheen, “Ridesharing.”
[In] 100 cars…six carry another commuter: Polzin, Steven. “The Decline of Carpooling.” Planetizen. February 25, 2015.
BlaBlaCar…share long-distance trips: Buhr, Sarah. “France’s Singular Unicorn BlaBlaCar’s Secret to Scaling Fast Globally.” TechCrunch. December 8, 2015.
China…[Uber] pooled trips: Manjoo, Farhad. “Car-Pooling Helps Uber Go the Extra Mile.” New York Times. March 30, 2016.
Waze…for carpooling: Alba, Davey. “Google’s Waze Launches a Ridesharing Service in Israel.” WIRED. July 6, 2015.
Lyft…commuting feature: Siddiqui, Faiz. “Lyft Ditches Casual Carpooling, Citing a Lack of Driver Interest.” Washington Post. August 24, 2016.
Putnam…“bowling alone”: Putnam, Robert D. Bowling Alone: The Collapse and Revival of American Community. New York: Simon & Schuster, 2000.
Project Drawdown defines the ridesharing solution as: increased ridesharing when commuting in North America. This solution replaces the conventional practice of commuting in single-occupancy vehicles.
This research focuses its analysis on North America (defined as the US and Canada), where carpooling and ridesharing are most common and where car ownership and single-occupancy vehicle commuting rates are high. Ridesharing and carpooling are interpreted to be the same for this analysis: “the sharing of a car trip by the driver with a non-driver.” The terms are therefore used interchangeably. Although most Americans commute to work in personal vehicles, the percentage that carpools is relatively small, at 10 percent. This percentage is used for both countries. The fact that few people carpool to work means that there is high growth potential, if the benefits of ridesharing can be properly understood.
In this study, carpooling is compared to single-occupancy car commuting. Cars represents the commuter mode for around 86 percent of all commuters in North America; hence, fewer than 9 percent of all commuters carpool (AASHTO, 2015; Toossi, 2012; Statistics Canada, 2011).
Total Addressable Market 
The total addressable market for carpooling is estimated as the total passenger-kilometers of commuting done by car in the US and Canada, using the average commuter driving distance.
Adoption Scenarios 
Impacts of increased adoption of ridesharing from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels. In each scenario, the average number of people in a carpooling trip increases from 2.31 to 2.5. 
- Plausible Scenario: This scenario projects that 15 percent of all car commuting is done by carpool in 2050.
- Drawdown Scenario: This scenario projects a more aggressive adoption, with 20 percent of all car commuting done by carpool in 2050.
- Optimum Scenario: The most optimistic case estimates 30 percent of all car commuting is by carpool in 2050.
As carpooling is defined here, there are no first costs to adopting the practice. However, an operating cost difference is included: with fewer cars on the road, the maintenance and fuel costs per passenger-kilometer are reduced, as they are assumed constant per vehicle-kilometer driven.  The average operating cost is divided by the average carpooling occupancy, which represents a 57 percent drop. The price of fuel is the average of the 10 years prior to our base year (2014), and comes from International Energy Agency (IEA) data.
Besides ensuring that the same car inputs were used across relevant Transport Sector models (e.g. emissions factors, fuel economy, etc.), no special integration changes were performed in this model.
The Plausible Scenario results in a total reduction of 320 million tons of carbon dioxide-equivalent greenhouse gases, and operating savings of US$186 billion.  The Drawdown Scenario shows a reduction of 360 million tons of emissions; in the Optimum Scenario, 3.2 billion tons of emissions are reduced.
The financial costs of commuting are motivators for carpooling, but history shows that increased carpooling is driven most forcefully by external forces rather than by policies designed to promote carpooling. Carpooling in America became popular during World War II, when the federal government asked people to conserve resources for the war effort, and in the 1970s, in response to oil crises that sharply increased gasoline prices. Decreases in carpooling have occurred since then, despite gradual increases in fuel costs and the institution of policies, such as high-occupancy vehicle lanes, meant to encourage carpooling. Barriers to uptake include lack of flexibility and convenience and limited regulatory incentives. Motivators include economic, environmental, and social benefits: there is evidence that the latter two are becoming more important for the younger adult population.
Although the Drawdown model does not include the shift of transport mode share from single-occupancy vehicles to other options such as public transit, cycling or walking, it shows that carpooling can be part of a global shift towards more sustainable transport behavior. Behavior is often not discussed at climate change or sustainable transport fora—instead, the focus is often on technology—but with sufficient numbers, behavioral change can contribute to reducing greenhouse gas emissions.
 For more on the Total Addressable Market for the Transport Sector, click the Sector Summary: Transport link below.
 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 can happen with more 3-person and 4-person carpools.
 An additional person weighing 75 kilograms in a typical 1800-kilogram car increases the total weight transported by less than 4 percent, so wear and tear and fuel usage do not change much with more people.
 For more on Project Drawdown’s Transport Sector integration model, click the Sector Summary: Transport link below.
 All monetary values are presented in US2014$.
Full models and technical reports coming in late 2017.