Because your wallet isn't already fat enough, the San Francisco Municipal Transportation Authority is currently cooking up another universal payment card to give locals yet another way to feed parking meters or pay for cab fare. Since the Clipper card you probably already have isn't set up for cabs and parking, the new card is an MTA side project for now, although the Chronicle reports that the new card's services could get rolled in to the existing Clipper system.
Don't hold your breath waiting to pay for cab rides with your Clipper cash balance anytime soon though, the one-stop-shop card is notoriously bad when rolling out new agency partners and features. The MTA card, meanwhile, is not expected to be ready to go for two years, but if the taxi and parking features are popular the card could eventually be used for Muni rides as well. According to an SFMTA spokesperson, a citywide card would give Muni more flexibility with pricing discounted passes or charging extra for peak-hour rides.
