You may not be so motivated to pick up littered plastic bottles when you walk around the city, but if those plastic bottles paid for your ride on the metro, you might be more inclined to collect those bottles. That’s the idea behind a new scheme in Rome where residents can deposit plastic bottles at certain stations in exchange for metro credit.

Compactor recycling machines have been installed at a station on each of the three subway lines. Under the scheme, returning a plastic PET bottle between 0.5 liters and 2 liters will add a 5-cent credit to your metro account. A metro ride costs €1.50, meaning you’ll need to return 30 bottles to accrue enough credit for a ride.