I use Strava, but all the segments I have created are uphill, thereby making the speed on them slower. Also there is little training benefit to riding fast downhill compared to doing it the other way round. I just ride my local rides and if I improve then so be it. I certainly don't know where other riders created segments begin, so have no idea when to 'attack', and certainly don't think blasting downhill at 40mph + on a touring bike with a cotton cap is a good idea, I'll roll down at 20 ish and carry on with my gentlemanly journey.
I'm not a racer, and believe anyone trying to bag strava KOM's at the expense of ped/other cyclist's safety should join a local club (and get put in their place)