OK, unlike previous games, I just played events and didn't deliberately drive any roads. I easily got to 510+ with only a few detours. Every once and a while when I was given a route by Anna if I was driving past a road I hadn't driven that was on my way I'd drive it. Also, if I went past a very short road I knew I might miss later I drove it.
When I was so close to 531 I then got a little more deliberate about finishing some roads when I was in the area. In my case, they were mostly roads south of the wind farm. I then was at 528. In about 1/2 hour I found two more roads that I had missed and was stuck on 530. I spent at least two hours trying to find the last road. I zoomed in tight and sat close to my big screen. Nothing spotted.
Finally today I got the last road just driving around. North of the eastern boundary of the Festival site about 1/2 between it and the Slate Quarry title is a little reverse J road. It was highlighted as having been driven. I had completely ignored, however, today while driving on the rotating map I noticed a tiny bit of grey. That was IT!
On the top map view, it did NOT look grey, however, as I was driving east on the road it's connected to the rotating map showed the grey. I think that road get's marked as mostly driven as you drive by on the major road. I assumed I had driven it. Unless you are watching closely it's easy to miss that you didn't. I'll bet a LOT of people are missing the tip of that little reverse J
The gamer tag is accurate. It's how I've made a living for about 30 years, however I wish was racing cars instead. Sigh