Forza Horizon 2

Its something a gamehead would do lol turn your tv color off and the lines will brighten up and darken up so now you can easily see it better

Although this achievement sounds like a pain in the ass (and looks it by the maps perspective) it's not really all that hard. There are a total of 315 roads on the map which require you drive down all the way Untill the greyed out line on your map becomes white in Colour.
You are also required to drive down the orange faded shortcut lines turns bright orange for this achievement also!

Driving around looking for boards, barns, speed traps bucket list events and just normal races should unlock most of these roads anyway just by working towards other achievements :)
Another thing that you should also consider is driving back and forth along each road twice as you can miss a little section of the road and it can become hard to find as the white will make it look like there is no grey lines.

Note: if you use while in the map, you can filter it so that all the icons are turned off making it easier so you can see the roads have and don't. (Credit Goes To: ScuzzyBunny)

Note 2: a tip by Hanrec2 pointed out that sometimes roads may (in their case highway) did not join smoothly with the other roads which took going back and driving carefully along them to get them joining and unlock the achievement.

Just did this yesterday, i missed 2 sections of road 1. at the airport, the 2nd one was a bit of pain as it was right at the edge of the off ramp for the motorway/freeway covered by the yellow central road might have been about 1-2 hours to find so make sure you check every road before moving on to the next one. Good luck

All above guides are good, but i suggest to start from other trick (this solution is only getting all advices to one and sharing my experience I waste more than 2 hour of searching - but first trick helped me right away and i found last road in sisterion center). So before you start "blind" driving on freeway and other places:
1) Enter game options, graphic and turn brightness to minimum (this is in game, not TV as 1 solution says).
2) Then enter your map, press and uncheck all (to clear map from icons (Credit Goes To: ScuzzyBunny and xTGE).
On that point, scan whole map - all roads shoud be continous lines, so those which are not, there are your missing ones.
If you still are unable to find last missing roads, you will missing one of 3 things:
A) Check Horizon Festival Hub (under icon which cannot be hidden).
B) Check airport - some guys have problems there.
C) Drive all freeways with there entrances (under brigde, each direction).

Hope helps and save some time to other. Good Luck:)

Top-voted solution is great for any of the regular roads that turn white when discovered, and helped me find one at theairport I missed. But turning the color down actually makes it HARDER to see bits of the offroad dirt paths, because they turn from pale orange to darker orange
Removing the color actually hinders spotting them. So if you're stuck even after turning color down, do the opposite. Crank the color up to the maximum setting on your TV to over-saturate the image. This should make roads you missed that turn 'orange' after finding them stand out like a sore thumb. Good example here, just left of my cursor was a loop that popped out bright as day with over-saturated color.

https://account.xbox.com/en-us/screenshot/93d117fc-1b9a-478b...

For regular roads, pay special attention to an places where one road passes underneath an overpass. You have to drive over the overpass as well as under it to discover both road portions.