system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do they monitor speed by aircraft on highways?

I did a lot of driving recently and saw this sign frequently that says something like Speed Enforced by Aircraft.  How does that work?
gpt
There are marks painted on the pavement a known distance apart. The vehicle in question is timed from when it crosses one mark until it crosses the next mark. Divide the distance between marks by the time measured and you get speed. 

Then they radio down the details.