Two achievements for the price of one. Here's how to do it. I advise saving your game beforehand...
1. First of all, mark this location on your map.
2. Start a mission to ensure the score for this achievement registers.
3. Prepare. Ensure that you get the angle on the curve. Basically, using a car with decent speed, you want to gun up the highway and approach the edge of this ramp at full speed and smash through the barrier. This will take you off the ramp, down to the green beneath and underneath the map.
4. As the score starts to go up, keep
held down until you're confident you have enough score. I held it down until 7,000. Releasing the trigger will unlock the achievement. You will also unlock Hypersonic
for this too. Double whammy indeed!