You could have a laser that moves rapidly back and forth in the vertical plane that would cover a wide range of height. I'm sure you've seen something similar at a laser light show. The trick would be the detector on the other end, but since it's only a maximum of a few feet it shouldn't be too difficult. Think of something similar to punch press safety curtain lights, only with a laser.
If something tripped the beam and it wasn't either car crossing the center line then it should be blatantly obvious it was debris. The light would only be used to make a decision in a situation like Neff when you know he's damn close to the line but don't know if he went over or not.