If you live in Washington D.C you might have heard about slug lines: the casual carpools that get commuters from Virginia into and out of the capital. They sprung up in the 1970s after the first high occupancy vehicle lanes (and lower speed limits) were introduced by the government in an attempt to reduce America’s gas consumption and its reliance on foreign oil.

Commuters wanting to use the HOV lanes would pull up at the bus stops on the way to the freeway and ask the people waiting in line if they wanted a ride into town. Unsurprisingly, for many passengers, a fast, free trip to work beat queuing for the bus. And the slug lines were born.

Today, slug lines work well in places where you have a critical mass of people headed the same way and an easy meeting place for drivers to pick up passengers. It’s estimated, for example, that nearly 10,000 people use casual carpools to get across the Bay Bridge from Berkeley and Oakland to San Francisco every day.

However, it’s much harder to make this work across cities because it’s tough for people headed the same way at the same time to connect with one another. In fact, we ran some numbers to figure out just how hard. If your home is in San Francisco’s Castro district and you work on Market Street (a three mile trip), you’d have to call almost 6,000 people to find someone who lived near you and was heading into the office at the same time. Even after making all of those calls, there’s still the possibility that you might not find someone who could share a ride.    

The good news is that technology has enabled us to solve this problem. Today, ridesharing apps like Uber can match you instantaneously with people headed the same way at the same time. It’s good for passengers because they pay less for a shared ride. For drivers, there’s less downtime between rides: our longest uberPOOL trip in San Francisco was over 2 hours. And getting more people into fewer cars means less congestion and pollution for cities.  

uberPOOL is a proven model. Since we launched in August 2014, over 100 million rides have been taken. Over 100,000 people are taking pooled trips every week in over 11 cities globally, including New York, Los Angeles, Beijing, Chengdu, and Shanghai. And in China, the number of uberPOOL trips has grown to over 20 million a month. So far in 2016, if Uber riders had driven alone instead of sharing their rides we estimate that over 90 million more miles would have been travelled — consuming 1.8 millions of gallons of gas and emitting 16,000 metric tons of carbon dioxide. That’s the distance from Earth to the Sun.

This is great progress, but there’s more we can do. So today, to celebrate Earth Day, we’re beginning to roll out uberPOOL to developers across the world — enabling carpooling to be an option from within their apps at the push of a button.* For example, Transit App, Live Nation, Foursquare via Button, and more will give you the option to request uberPOOL.  In addition, we’re supporting the theme of Earth Day and the goal to plant 50,000 more trees. Today in Los Angeles, San Francisco, Washington D.C. (for a full list of cities see below), when passengers choose uberPOOL and enter the code EARTHDAY16, we will donate $1 to the Arbor Day Foundation to plant a tree in their honor.

Oh yes, and you were probably wondering what slugs have to do with casual carpools. It goes back to Virginia. Bus drivers were trained to look for counterfeit coins, known as slugs. And they started using the same term for passengers who appeared to be waiting in line for the bus, but actually wanted to carpool. From slugs came slugging and slug lines.  

Participating cities include: Atlanta, Boston, Chicago, Los Angeles, Miami, New Jersey, Philadelphia, San Francisco and Washington DC.

*If you’re an Uber developer, follow Uber Developers on Medium to be the first to know when the uberPOOL support is fully rolled out. Check out our uberPOOL documentation to get a preview of what’s coming.