As someone who nearly got run over by fellow MTBers after crashing at a trail centre, even at trail centres you should not go so fast you can't stop in the distance you see…
I hit a bloke at Swinley last year. I was enjoying myself hugely ripping along the singletrack, when suddenly there's a bloke coming the other way going at a far more sensible speed. I thought there was going to be a major accident but we somehow managed to avoid each other except my brake lever smacked his little finger drawing a bit of blood. Entirely my fault, and I felt pretty bad about it even though he was ok with it really.
Now I go slow on the really blind bits and I tend to shout 'rider' as I approach stuff just in case. I met someone coming the other way doing the same at night, and I could hear him from miles off. As we passed he mused that it was a bit pointless since we were all lit up like christmas trees anyway and were visible from some distance away.