You can earn this achievement at any time during the game. However, in my case, I have found that the best place to do this without much fuss is in the airfield area during freeplay or any case. The choice is yours.
The area is north of Beverly Blvd. Please see attached photo for the exact location.
The crosshair points to one of the entry gates. There is a straight line to drive through and you are surely going to be going over 80mph for more than 10 secs straight. You can also drive over the grass if you like.
Moreover, there are a bunch of cars usually parked behind the building so you can change your car if need be.
Hope this helps. Happy hunting and Godspeed!