HMM: Waze thinks it can get Americans carpooling again.

Waze says carpool customers completed more than 550,000 rides globally last September. (Waze Carpool is available in the US, Mexico, Brazil, and Israel.) The company predicts that it will cross 1 million monthly rides by early 2020. Carpool customers in the US collectively drove 25 million miles last year, which the company estimates helped reduce carbon emissions by 20 million pounds, thanks to combined rides. But Waze would not reveal the total number of people who are using the app, nor would it comment on its retention and turnover rates.

“We have what we think is a magic number,” Josh Fried, head of Waze Carpool, said in an interview. “And that is four rides… If we get you into one carpool, and then you end up completing four carpools, you stick [with the service].”

Better than most light rail systems, I bet.