That's a great idea that I was also thinking about when I first read about this story. I'm not aware of anyone who has created a service or app like this, but something that leverages the power of crowdsourcing could create a lot of useful info/advice. Not just about "dangerous" neighborhoods but even to avoid routes/areas with a lot of traffic congestion, bad potholes, etc. The concept could certainly be expanded to include more positive things, like finding bike-friendly routes through a city or finding streets that are good for hanging out in during the evenings, etc.

Kind of like taking the Yelp concept to something more visual, on a map. Say, the particular topic is "Streets with potholes," users could click on particular city blocks or stretches of road that are heavily rutted on a map to indicate places to avoid. When calculating a GPS route somewhere, it can take that user-input info into consideration.

Of course, depending too much on user-input has its pitfalls, too. Such as a map that keeps showing a pothole-filled street even though it has been repaired because no one bothers to update the status of that spot.