A percentage-based tip scales with the prices on the menu, which are closely associated with the cost of living in a particular area. A 15% tip is exactly as appropriate in 2016 as it was in 1916. It works as well in Florence, Alabama, as it does in Baltimore, Maryland.

The only reason to increase the percentage of table service tips is if the restaurant employee does more for the customer. I have not observed this to be the case in the last decade, in larger cities or out of them.

As logical as that might sound the tipping percentage is a matter of cultural norm and not about maintaining a status quo. In metropolitan areas 20% has long since been the standard and is arguably increasing. The rest of the country has been steadily catching up.