That said, I just don't use signatures unless required to by corporate policy.
For some people, 'have a nice day' is code for 'f you'. Dropping your kid off and they're being a pest, but you don't want to drop an f-bomb? Tell them to 'have a nice day'. Your parent is being a pest while dropping you off? Tell them to 'have a nice day'.
Then it's a little weird when a snack machine says it, and kind of fitting when it's on the front of the Facebook commute busses.
'Have a good day' is totally fine though. I mean it's still gonna upset the people who had other plans, but at least it's not rude.
"Thank you, but I have other plans."