(You can drive outside the roads using either the wallhack or a non-patched glitch spot (currently the only one which still seems to work is this:
Please note that, in order to be able to drive at full speed, without spinning out of control, once you exited safehouse properly, the back wheels of your car must touch the ground near the fence before you go in "the void". If you reach a road that's below you, you will fall on it, except when you are using wallhack). Don't deactivate the wallhack or enter/exit safehouse when you are in the void - you will be stuck. Also, when you are about to use the wallhack, don't accelerate too much or enter in an awkward angle, as you might end up spinning.)
There are a few more current locations that I haven't covered in these videos: Lower Rockport West Side (the locked connector area between Palmont and Rockport), the mysterious pink marker, the small turnpike highway, the funny garage floor texture from Agostini Heights, the aqueduct ends (you may know the main aqueduct area from Waterfront Road, its ends are out of level but have driveable surface!), the lakeside house near Diamond Hills, the showroom cilinder and safehouse (there are also other things which don't exist anymore in the game, like the beta safehouses). I may make a map of all these soon.