Carry a $5 bill when you ride.

It will buy some food if you bonk hard.
It will boot a torn tire sidewall.
It is enough to bribe someone in a car for gas money if you break down, and is better than carrying a $10 or $20 as they are guaranteed to "not have change".
You aren't likely to find an impulse buy along your route that a $5 bill will cover, sparing you spending it.