The achievement also popped at 529/531 for me. 530 was the very next road, but it took me another couple of days to find that last section of missing road. Couldn’t see it at all on the map, then while bombing across fields to a Forzathon Live event I must have gone close enough to a road (definitely wasn’t on one) and the counter popped up to say 531/531.
I was finding last road for about half an hour only to find small road in Edinburgh… 
530/531. One more “bit” of road to find.
Did you look at the little reverse J I mentioned earlier in this thread?
I got this last night. Unlocked at 529/531 like others have mentioned. Which wasn’t a big deal for me, because one of them I already knew where it was, and the other was a pretty big stretch that I was able to find, though I’m not sure how I overlooked it the first time through.
Stuck at 526/531 I can’t find any more. H E L P
Open map, turn off all icons, zoom all the way in, and slowly comb over the map. Also be sure to read all helpful tips in this thread. 
I got the achievement at 530/531 roads. There is no road left. When I was around 527, I couldnt see any undriven roads, however I realised I havent driven the beach drag strip, when I did that I carried on till u reach the edges of the skill areas near the castle. That unlocked 3 roads and achievement came.
But I Dont see road #531 anywhere. FH3 you had to drive all the roads to unlock this but not in fh4
Do you look at the little reverse J road I mentioned earlier in this thread? It’s 1/2 way between the festival site and the quarry on an east/west road. It looks red from the map view because as you drive past it most of it is marked as driven, but unless you drive up to that road and look on your rotating map there is a very tiny sliver of grey. It’s tricked a lot of people, including me.
yup, did check tht , i had already driven that.
I don’t know if this will help, but this was the field I was in the middle of when my “530/531 roads driven” message popped up.
My last two roads were in the Mudkickers park (one of the jump run-ups) and over in the middle of the Roman Ruins, both of which I had scanned over multiple times without noticing. Don’t be afraid to re-visit areas you’re 100% sure you’ve cleared, especially after you’d had time to reset your eyes/brain.
I say that because I’m pretty sure I’ve every inch of road on the map and it says I’m short at 527 roads.
Yes, they are all there.
Turn off all icons on the map and look closely. They can be hard to see
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Yes. Achievement popped for me at 529 of 531 roads found on two accounts.
On the latest account as soon as it popped, I used a blank map and scoured it. Found a curved tip at an end point of a dotted orange lined road that hadn’t been touched, that section was completely gray. That got me to 530/531.
Looked over map again, over and over about 10 times before finding a second slightly curved tip that was shaded half orange half gray. Once I covered it completely, 531/531 flashed up
100% definitely. I had the same concern when I first got the achievement, but there absolutely are 531 roads. Keep at it!
More info: https://forums.forza.net/turn10_postst132386_The-Noisy-Cartographer.aspx
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I got the achievement at 529, I have 530 now. No clue where the last one is. I’ve looked at the map until im crossed eyed right next to the tv and see everything done, so damn annoying!
Pay extra close attention to the dashed orange lines. Took me about 10 close up viewings on my 65" tv and I’d found one ending dash that actually began as orange and faded to gray. Very difficult to see without going at it one road at a time.
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Just want to add that my final road was one of the drive ways leading to one of the player houses. I had stopped just short to enter the house and a tiny grey dot was missing at the end.
So check those too.
Got to 531 a couple of days ago!
I noticed three of the four missing roads because it happened to be zoomed into that part of the map on the menu screen…three small bits of grey in Mudkickers.
Last road was a tiny spot of grey I noticed on the beach.