I stumbled upon a fantastic archive of old aerial imagery. It goes back to the 1920s, and has many, many images, at least from the urban areas of LA and the San Gabriels. If somebody wants to research old trails, or just see what their neighborhood looked like back in the day, it's quite nice:
Basically the map shows the brightest routes as routes most travelled and the dim routes as routes less travelled. This should be a great tool for all of you trying to plan your next adventure to somewhere not crowded. I found new routes not found on maps. Check it out!