If you do a search on "Stop and go with mercy", you'll discover that this is a VeggieTales song ("On the Road with Bob and Larry", "Jonah") that features in various family households. Ours isn't exempt, since we have worked through "On the Road" on several car journeys.
Our youngest (5) is a little shaky on the words, though. This morning she's been singing "Stop and Go with Percy". I'm not sure who she thinks Percy is - perhaps somebody in the van with Bob and Larry, along with "all those children in the back"!