Always look busy. If you smile and/or wave at someone while walking fast and either digging around in a bag or actively drinking out of a water bottle (ideally a metal one or something similarly clunky- a plastic water bottle isn’t that unpleasant to drink from while walking, so doing so does not indicate a rush as clearly), they feel greeted, but it’s clear you’re not in a good place for a chat.

You can also pretend to be on hold on the phone, but they’ll think you’re a real dick if your phone rings unless you can play it off as a disconnection/auto redial.