If they're not making weapons for fairies they're doing nasty things to people. I have never liked the look of those things. They seem to have exposed blades that wouldn't be allowed in a domestic garden but are alright on the open road.
Something like this spinning at speed. There should be signs out saying hedge cutting in progress. Had these been missed by the cyclist or were they not put out by the tractor driver?
Generally the cutting part should be on the near side... so I don't see the risk and any need to have "hedge cutting" signs. Usually the sign of new cuttings on the road is enough to make you take notice.
As for the accident, I can't imagine how that occured.