Forza Horizon

I would add : use the filter and "clear" the map

I was missing one road, and i didn't find it .... i clear the map and the road missing was a little part of road near a showcase that i didn't participate ...
Look the access of highway I-70 : there are 4 road on each access

On your Map the roads not yet driven are GRAY. When you drive on them they change color to white, yellow, or brown.

Use your mini-map on your HUD when driving on gray roads to easily find other undriven gray roads nearby.

UPDATE: When I had only a few non-driven roads left, I noticed that the remaining undriven roads would appear on the map as gray one at a time as I completed each road.

For me the last ones were the freeway off-ramps. There are 3*2 in total i believe. Make sure you hit these as they can be a bit of a pain to find on the map.

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)

If you're missing 1 try here... NW of the map is a place called Carson. Just south of the letter 'R' of Carson, zoom the map and you'll see a park which looks like a circle with an 'X' through it. The road running north/south on the right hand side of this park has a tiny bump on it's right side.... this is the entrance to a multi-storey car-park and the Carson Road Racing events. It was the last road missing for me, and seemed very missable, especially if you're attempting this achievement early in the game before you unlock some events, so i hope this helps some of you guys too. Happy driving.

I haven't gotten this yet but it wo'nt be too hard - I'll do that I did for FH2 on the one - simply use the filter and turn everything to off, then on your TV change it to black and white - normally by reducing the colour, then turn the contrast up - all the roads you haven't yet used are black, and the rest are white!