alt Hacker NewsTo know that you've used transit.
Not sure how it works in SF, but many cities across the world have moved to "in-card tickets." You tap your card on the way in, and the bus "knows" that this card number has a valid ticket attached to it. Depending on the particulars of the city and system, you might need to tap out too.
In some cities, this is done entirely offline. For cards which cannot guarantee offline payments (which many low-income / under-age individuals have, and such people are far more likely to use transit), this is done "on trust", with failed payments getting your card blacklisted until you go to the transit agency's office and get the situation rectified.
> with failed payments getting your card blacklisted until you go to the transit agency's office and get the situation rectified.
At least in the NYC MTA's OMNY, unblocking actually happens automatically in the background, either on a nightly reconciliation job that attempts to collect your fare or triggered by the next tap (which will be unsuccessful once more but trigger the collection attempt, and then you can tap again after your card's blocked status has cleared in the system). They also have a website to manually repay without having to wait at the turnstile.