system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How are speed limits enforced by aircraft?
gpt
In areas where speeds are enforced by aircraft, there will be timing marks painted on the roadway. An officer in an aircraft overhead will time cars as they pass between these marks, and if a car crosses the gap in less than the benchmark time, they will radio down to a car waiting on the ground, which will make the stop and issue the ticket.

But this is expensive, as it requires at least three officers (two overhead and one on the ground), so it may not be used very often even when the road is set up for it.