You know that if it says, "Adopt a Highway" and no one does, we're driving on orphans.
Highway to Orphansville
Explanation
Explain Bear
Listen here, chucklehead. You think roads are just there? Nah, they need maintainin'. The joke here implies if folks don't "adopt a highway" (which is a program where volunteers keep a stretch of road clean), then roads are built outta somethin' else... like orphans. It's funny 'cause it's messed up and not true, you dummy.