For this achievement you need to drive every road marked on the map. There are some small back-alleys which are not marked on the map and do not count towards this achievement or your "Roads Driven" counter.
You can check your progress by pausing the game, selecting My Profile then Stats and pressing RB to view the Discovery tab.
You may find that you're left with a few undiscovered roads that are difficult to find. Use these tips to help you find the remaining undiscovered roads:
- Obviously zoom in as far as possible and slowly scroll the map. Also, use the filters (Left Bumper) to remove all of the map icons.
- Look at the "triangular junctions" as it's easy to miss one side of these (there are in the Carson area)
- Pay particular attention to the freeway slip roads
- Drive underneath all of the freeway junctions (west side of the map) - these are partially obscured as the roads cross eachother
- Re-drive all roads which come to a dead end. These can be found at Redfoot Ranch (south), Carson, Eagle Ridge (south near the Foothills)