JEAN KNAACK couldn’t keep a lid on it. While on a six-mile run near her home in Maryland, she raised her water bottle and expelled its contents onto the passenger-side window of a car.

Ms. Knaack, a 115-pound runner, had been jogging on the sidewalk when the vehicle had come within inches of hitting her. The driver had blindly pulled out of an adjacent parking lot, and Ms. Knaack responded with the aggressive squirt, coupled with a few choice expletives.

She did not anticipate what happened next.

The driver pulled the rest of the way out of the parking lot and into the street, whipped around in an intersection, got out of the car, and confronted her. Amid of flurry of profanities, the motorist threatened to strike her with a beer bottle. “The fact that he was so specific really scared me,” she said. “My heart rate shot sky high. I felt like I was going to pass out.”

Even though Ms. Knaack was a seasoned runner  she’s the executive director of the Road Runners Club of America  and is knowledgeable about proper training technique and nutrition, she never got the memo on what do when an angry or negligent motorist takes a workout sideways. That’s because there really isn’t one.