It seems the total required distance is much smaller than other guides have suggested. I got this near the beginning of the game just grabbing people for the Love Thy Neighbor achievement, and having fun with getting rid of them to grab the next guy.
Get to the westernmost bridge running between the two islands (just east of the prison island) out near the center where it becomes four lanes. Stop a car on one of the outer lanes here, grab the driver, and toss them off the bridge. Likely the quickest and easiest method because there is no required story progression to get here:
It seems if the toss is 100+ feet, you will get the achievement. I received mine at 122 feet.