Oh that's interesting, I actually thought about that recently when I saw a "speed monitored by aircraft" sign on a highway. I wondered if an ADS-B receiver wou

Oh that's interesting, I actually thought about that recently when I saw a "speed monitored by aircraft" sign on a highway. I wondered if an ADS-B receiver would pick it up. But... indeed, it's quite often that I see an aircraft and observe that it is not being picked up by my ADS-B receiver. Not sure whether due to low altitude , or that they are simply not transmitting..