[ Register or Signin to view external links. ]

This achievement can be done at any time during your playthrough; all that's required is a fast sports car. Below are three of the best cars to use:

De'Leo Stiletto - randomly found in traffic in Frisco Fields; very rare (this car is used in the video guide below).
Samson Duke - randomly in traffic; can show up anywhere (only fast enough with tuning upgrades).
Samson Opus - randomly, though rarely, found in traffic; can also be found here.

Once you have the proper car, the route you'll want to take goes from south-west Frisco Fields, over the bridge to Barclay Mills, through the tunnel and down the main road. It's a fairly straight road to maneuver, and since you'll be heading downhill after the bridge, maintaining top speed will be easy. Just make sure to steer accordingly through traffic as to not hit any cars.

If you're able to maintain a speed of 120mph for twenty seconds, the achievement will unlock once you begin to decelerate.