This is very easy but takes some time to figure out when the best time to get this is. Don't do this at night when the Freaks are out because all the civilian vehicles are gone and don't jump on Cell or Agency vehicles because they stop often. Find a car that is just driving around and once you see another car that is coming the opposite way or driving next to you. Jump onto the car with . This may take a couple practice runs to figure out how far/hard you need to jump. You can also do this in co-op while playing with a friend. He can be driving a car around while you jump onto other vehicles. Make sure you are going faster because sometimes it doesn't count.
i got this achievement in a weird way, i was hapely walking in the streets, then suddenly one of my friends shows up with a cruiser and goes right at me, behind me was a friend in a chopper, i landed on the chopper and got the achievement. lol