"Three mailboxes and a piano, huh?" That has to be the start of a joke. The joke is everything in Sans's life that led him to this moment, probably. He takes a step down the sidewalk, then pauses. "You want me to tell you when you're three mailboxes back?"

Normally, Sans figures someone who's blind has enough experience to figure out where they are unless they explicitly ask for help, but this guy almost stumbled into the road, so Sans figures he should probably get that out of the way right now.