Just a reminder that the electronics to make a warning system are about $100. A software defined radio to accept the ADSB transmission and a Raspberry Pi to interpret and some display and a warning buzzer are cheap.
But the FAA refuses to mandate ADSB-OUT on all manned aircraft and mandate ADSB receivers that can provide a warning on all manned aircraft. It's not just day-one pilots getting their solo time in and being mowed down by careless twin pilots. This has happened over and over and over and the FAA sits on its hands. Except for unmanned R/C hobbyists, whom the FAA are regulating out of existence in spite of no fatalities in the last 50 years.