Hey Bob- we were genuinely worried about bike jacking when we were planning the trails. However, in the year and a half that the trails have been open, we've had zero reports of assault, bike jacking or the like. In fact, crime has actually dropped around the park, as riders have displaced the homeless camps that used to define that section of the park, and the 34th precinct has seen less car break-ins around the park as a result. I think most bike thieves in NYC prefer to steal locked-up bikes.
There is a sketchy area around the I-95 bridge that seems to be a crackhead hangout (the cops know about it and patrol it, but there's only so much they can do... there's a methadone clinic across the street and the guys get their morning high there, then hit the park for crack), but the trails are all well north of that. I do sometimes loop down there (last year's race course went through that area), and have never had a problem- I just ride fast and don't stop.
I love riding Highbridge. It's what you'd expect for mountain bike trails in Washington Heights- gritty, tough, sketchy in spots, unforgiving, duurrty. They're not pristine suburban trails, because it's NYC. I ride over glass every day on my commute into Manhattan from Queens, so it doesn't bother me. It is what it is, and we take the long-view with it. In 5-10 years as the neighborhood continues to turn around and clean up, the trails will only get better. 10 year ago police and park staff would barely set foot in the park. We've come a long way and will continue to make progress over the next 10 years...