FLARM
I thought it might be useful as people are talking about collision avoidance systems to give you my experience of FLARM
I have installed FLARM and also I am ADS-B out. This is just my experience of GA aircraft.
I have a Eurostar which has good visibility but on many occasions I have had alarms about other transponder equipped aircraft sometimes behind me or ahead which I had not yet seen.
With ADS-B out - (which is very cheap to install if you have a gps cost me just £12 for a cable but the process with the LAA and BMAA is currently too complicated needing access to a PilotAware system) other systems can see you for example PilotAware.
With FLARM if the other transponder equipped aircraft has Mode C it will also tell me if the aircraft is above, below or at the same level. Also it computes a rough distance from me and then lets me know if the source is getting closer or moving away. If the other aircraft is ADS-B out (which some are) it shows me where to look for the other aircraft.
It is a bit expensive but very cheap if it saves you once. Much cheaper than a funeral. After a years use I would recommend it and something like this should be mandated for GA as FLARM is in many worldwide gliding environments.