Having just recently experienced this, at 15-20 mph he was well out of easy speech range before I fully realized what had happened, let alone had composed a reasonably polite thing to say.
20 mph is 30 feet per second, and this yahoo brushed my back as I was talking to someone. 5 seconds to turn, understand, and start talking isn't unreasonable. If you can politely get the attention of somebody 150 feet away and receding rapidly using your voice, I'd sure like to see it.
