As for the $30 point, that may be where a lot of flat fares go - airport for $30, for instance, is probably a common data point.<p>As for the rest, if we're looking at cash fares, doesn't it seem necessary to examine the most common makeups of bills that would get you to that amount? If someone has all 20s, it's easy to tip $10 on a $50 fare. Boom, done. But if it ticks over to $60, perhaps people don't want to break that fourth bill, or maybe they don't carry more than 3 20s, so rather than pull out the card they just leave no tip, like a jerk.<p>But I don't get why cash versus card was not broken out here. Surely tipping habits differ greatly, or at least significantly, between the two methods. Why a card user doesn't tip at $60 is likely to be different to why a cash user doesn't.