Alright, the achievement is quite self explanatory, but I would like to give some helpful tips to make this achievement easier for you.
First of all, this achievement requires you to discover 141 roads and is far easier than that of the one in the base game. Most of these roads will be discovered through you just racing in the game. That is why I would recommend you finish off all the races in the DLC before just going out of your way to grind this. Doing this allows you to discover 100+ roads in the process. When I was done with all my races, I had already discovered around 125 roads.
You can check your entire progress by pressing the start button on your controller and tabbing over to the progress bar of the menu. When you are there, select your stats and go all the way to the far right to get to "Hot Wheels." There, you can see all your DLC stats, including the roads discovered.
As stated before, finding the roads is pretty easy in this DLC, but I am going to add some images of the last few areas that had roads to be discovered for me.
This was the last road I discovered
and it was kinda hard to see due to it being inside of the loop. If you are still looking for 1 or two more roads, look at the loop a loops.
Here was another area
of the last remaining roads.
And here is one last area
that may contain some roads that you may still need. Its in the bottom left corner and its the main city, so its that area with the smallest sized roads.
Hope I helped out and ff you downvote, please let me know in the comments so I can add or change something. Cheers! Edit: Lol sorry for how the images look. It was the best way I could think of circling the areas I wanted to highlight.