I have no facts to offer this debate. I do, however, have an opinion.
If cab drivers are not obligated to make change, then why would they ever offer change? If you can charge $20 for a $15 trip, why charge $15?
The obligation should be on the cab driver to carry change, to a point. Personally, I would expect to see cab drivers required to advertise that they can make change to, say, $20, and if they aren’t able to cover it, then the customer should get a discount. Charging extra for the service because the driver doesn’t have change doesn’t make sense, especially as you pay for a service you’ve already received.
I live in Australia. We have a higher minimum wage, therefore we don’t need to tip. Some people are starting to put out tip jars due to the influence of American culture, but I don’t tip because that would be double-dipping. Their wages are factored into their prices.
I frequently catch taxis around my town. They all have a clear and easy to read trip meter that the passenger can see from anywhere in the car. They all have a list of rights and obligations posted in plain view that states that the driver must take the most direct and practical route unless another is specified. The list also tells you how much you are to be charged for each service (hiring fee, per kilometre fee, late night surcharge, etc). I have never had a cab driver struggle to produce change for me. In fact, many of the local cab drivers will pause the meter when it reaches an even dollar figure, because that’s the kind of guys they are. In return, I never pay them with large bills, and I try to include as much loose change as possible - they appreciate this.
If I had a pizza delivery man show up with a $14.55 pizza, and only a $5 note for change, I would refuse the pizza, call his boss and complain, and change pizza parlours. How dare you assume I should tip you 45 cents. When I get home delivery, I’m asked what denomination I’ll pay with over the phone and the delivery driver has the correct amount of change taped to the box when he arrives. In the rare cases that I haven’t been asked first and the driver doesn’t have the right change, I’ve been given a discount rather than overcharged. The only delivery person I’ve ever seen try to get a tip by carrying the wrong change is the grotty guy from one of the local fish and chip shops. We made him come up with the right change - after all, the fish and chip shops charge a home delivery fee! He’s slow, which is weird because he drives like a maniac, his car is so loud it wakes the whole neighbourhood and he often brings his weird friends along - he’s not exactly a tipping candidate. We changed fish and chip shops after the tip incident.