A runner comes up behind you. Your dog notices the movement, barks, and lunges just as the person passes. You cannot ask the runner to be part of a training session, and you don't need to decide in that moment whether your dog is frightened, excited, or wants to chase.
Choose a wider, quieter route while you build a plan. Keep your dog on a secure lead and leave room for runners to pass without getting close. When you hear someone approaching, move aside or turn into a calmer path before your dog locks on. Avoid standing at the edge of traffic to make space.
Work with a slower picture
Ask a helper to walk at a distance in an open, safe place. Reward your dog for noticing the person and being able to look back at you. Once that is easy, ask the helper to walk faster, then jog briefly. Change just one thing at a time: speed, distance, or duration. If your dog freezes, stiffens, refuses a usual treat, or barks, go back to an easier version.
Teach a cheerful U-turn away from runners first when none are around. That gives you an exit on real walks, where you cannot control who comes by. Don't lure your dog closer to a runner with food, and don't let a chase start. A safe pass at a distance is success even if your dog never sits perfectly still.
If your dog has reached a runner, pulled you over, or redirected onto you, ask a qualified, reward-based behavior professional for a safety plan. Sudden changes or signs of pain warrant a veterinary check. You can work on the training without expecting the public to slow down for it. For more on responding to fast movement, see the movement-and-chasing guide, runners should never be used as unwilling training partners
Sources: Dogs Trust: How to stop your dog chasing cars, bikes and animals. Cornell University College of Veterinary Medicine: Managing reactive behavior.