The New York City Half Marathon, which took place in March, had its own road-related requirements. The last wave of runners had a little less than two hours to run six or so miles before reaching the F.D.R. Drive, and if they went any slower, they were forced to leave. In the end, about 10 people were directed off the course at the Houston Street and 23rd Street exits so that the city could get the F.D.R. Drive — a desperately busy road even in the best of times — back, said a spokesman for New York Road Runners, the race organizer.

Similarly, administrators in some trail races that follow multiple loops through the woods bar the slower runners — the ones unable to complete the first loop in a set amount of time — from even attempting a second loop.

It’s not always an exact science. In the case of the Seven Mile Bridge Run, any runners overtaken by a shag bus are considered by definition to be too slow to finish the race. They have no choice but to board. It is a ruling with no chance of appeal.

This year, the first group of unlucky runners was plucked off the bridge at about the 4.5-mile mark, with still more than a half-hour of racing time to go.

“There’s a lot of disappointment” and a lot of talk of “beating the bus,” said Janet Oechsle, a longtime volunteer. She recalled an earlier race in which fear of being snaffled by the bus inspired a group of women to purchase shirts printed with a tricked-out version of the slogan: “Beat the Damn Bus.’”

Raceday was hot and humid. From inside the bus, the slower runners seemed like meek prey being hunted by oversized predators ready to pounce on their weaknesses. They fanned out, loping off on their own as if they were scattering across the plains (or wherever it is that prey scatters), but resistance was futile.