Stunt RiderG 20

There are a couple areas you can unlock this achievement.
Harder way: Just like the Stunt Driver achievement, you can unlock it at the entrance to the Maintenance Tunnels however, I tried several times with the motorcycle and was not able to but it is possible. Jump in the convertible and clear a path to the entrance. Try to make a clear path because you will need all the speed you can get and you wont have much. Get on the bike and back up all the way to the wall and floor it. If you're lucky you might clear a short distance and unlock the achievement during the loading screen once you enter the tunnel.
Easier way:
One way I found much easier was in Leisure Park at the top of the stairs at the entrance to North Plaza. Go grab the motorcycle by the Maintenance Tunnels and come back out the entrance to Leisure Park. Take a left and you will see a set of stairs leading up to North Plaza. Go to the top and back the bike all the way back to the right side of the top of the stairs. Don't back up too far or you'll fall off and have to drive back up or get stuck and will have have to grab another bike. Clear out all the enemies at the top of the stairs as driving through them will slow you down slightly. Drive off the side and the achievement should unlock.