Earlier quoted context omitted.
Tell me, step by step, how you think I will politely but effectively call out somebody going by me at 15-20 mph. Use diagrams if you need.
Use your voice
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.