When a driver rolls down the window, I take that as an invitation. Have a flexible time schedule. Respect safety of volunteers. For safety, Leon restricts the on-street work to adults. When families volunteer, the children stay busy at the parked van filling cups and snapping lids on them.
Safety is another reason they only work one-lane streets.
That way, volunteers can easily get refills during traffic light changes. Map out your strategy. Reveal your identity up front. Carrying four cups per tray, the volunteers fan out and take five-vehicle sections so they can reach as many drivers as possible. Look for other creative ways.