Two possible causes I can think of:
They've seen kids playing some sort of dare game, running across the carriageway
Or
There are workmen due to be going out and putting signs up indicating road works coming up. Those guys have to run across the motorway carrying those big triangular signs that sit in the central reservation. I know that if I had to do that, I'd want to be as certain as possible that traffic was slowing down.
In the first case, they may not know exactly where they were, in the second they will know where the road works is going to start, but may well not know how far along with putting signs/cones out they've got.
Although this does seem to be a very long stretch that you describe.