Test Drive Unlimited

Roads you have discovered are marked as blue on your map so just scout around and jump to places near your grey roads.

Tip: to see how many miles of road you still need to unlock go to your house and check this achievement, there are 1000 miles of road in the game so 1% = 10 miles.

Good luck.

When you start the game from scratch, all of the roads will be coloured grey meaning you haven't driven on them. As you play the game and drive on the roads, the colour will change to blue meaning that you have driven on it.
To get this achievement, you must drive on all of the roads and make sure all of them are coloured blue. This means you must drive on every single Motorway, Alley, Hill, Corner, Junction, and other roads supplied in the game. This will take awhile to do.

My suggestion would be to actually save this achievement for a later stage. As you play along, and do the Single-Player Events and Challenges, you will drive on roads, in the game you may have not driven on originally. A problem which can occur, is some events may not open up, until you have actually driven on the road which contains the event.

When you decide to do this achievement, I would suggest you start at a certain point on the Global Map, and then go around the whole island, in a circle until you return to the point, which you originally started at. I personally decided to start south, since it contained all of the city roads, and then I went west, south west, north west, north, center, north east, east, south east, south, and then finished around that point. It is totally up to you which way you wish to go to get this achievement done.

A strong tip I would actually suggest when doing this achievement is to use a Motorbike. The reason is because the Motorbike is more easier to use in getting the junctions done quicker, and effective, since you can squeeze through the vehicles which are in your way. To get a Motorbike in the game, you will have to find all of the Car Shops which are in the game. Motorbikes aren't costly, as they range from about $10,000 - $25,000 which isn't a lot in the game.
Just watch out for the police cars, because if you hit one and fall over, it's an automatic fine. And if you're at Champion Rank, it will be costly.

Another little tip I can provide is, if you go into a Home and then look at the Achievements page, and then select the Explorer Achievement, it will tell you how much out of 100% you have done towards this achievement. The strangest thing is that it will go up quite high, even though you may have quite a fair amount of roads, left to drive on.

Once you have driven on the final grey road in the game, you will unlock the achievement, and you'll be awarded a prize. I will not tell you the prize, you'll have to find that out yourself.