If you are driving on a route that another active (meaning they often report or thumbs up or not there other reports) wake user has been on in say the last 15-20 min your route is shown in green. If it's been 20 min - 1 hour your route is yellow. Longer than that shows a red route.
Example I was driving on I-10 around 2am in north Florida. I couldn't tell if my route was "safe" or I was just the only person who had been down that stretch of road in hours.