Whenever the fare goes over $25.00 and the passenger pays with credit card, a signature is automatically required. If I don't bring back a signed receipt for every one of those premium fares, the cashier at my taxi garage does not cash me out. Since the great majority of rides remain under that amount, many people have absolutely no awareness of the signature requirement. The result is that half of those times, my passengers blare out something like this:
"I have never heard of this before, having to sign a receipt, are you sure?"
"sign it? this is the first time I have EVER been asked to do that. how come now?"
"...is this a new thing?"
DEAR RIDING PUBLIC: YOU'VE BEEN ADVISED! ANYTHING OVER $25 REQUIRES A SIGNATURE AND NO, IT IS NOT SOME GIMMICK CLEVERLY INVENTED BY THE CABDRIVER FOR SOME SURREPTITIOUS PURPOSE.
Also, please please please stop freaking out when the driver asks you to swipe your card again.... and perhaps again, and again. The fear of being charged multiple times for each and every swipe is irrational. Once a single transaction goes through successfully, the meter automatically clears itself. There is absolutely no possible way for your card to be charged more than once for the same total fare amount. Sometimes the machine outright declines the transaction, for a number of reasons.
If you have no other functional cards, it is required of you to pay cash. That is when many people fall under the assumption that the driver is trying to collect cash on top of the already "charged" card. It is very simple to clarify this misunderstanding. Just ask for your receipt and make sure that it is the receipt that was the very last one to print out before being handed to you. You'll hear and see it printing. You'll hear and see the driver ripping it off and handing it to you. If the bottom of the receipt says something like....
CARD TYPE: VISA
....then the card was charged and it was charged ONLY ONCE!
If none of that is present on the receipt, then it was a CASH PAYMENT ONLY!
One last thing: You cannot pay for some of the fare amount on card and some on cash. You must pay the full amount due with either one or the other. Tipping is a separate issue. You can tip in cash after applying zero tip on the screen. Or not tip at all. And if you are one of those people who are perpetually afraid of cabdrivers ripping you off, please do me a favor and read about yourself here.