Crackdown 2

Jump on to any car, preferably a civilian car since Agency cars will often stop to fight Cell, and Cell cars will stop immediately to kill you. Even the civilian cars will stop for a moment once you jump on to them, but once they get moving, shoot the ground in front of the car. The person in the car will panic and drive faster for the next few seconds. Keep shooting periodically to keep them driving fast.

Once you see another car coming, do a small hop (don't hold A, just tap it) so you don't miss and land on the next car. It's best if the second car is coming towards you, so you land on the hood. The second car does not have to be going very fast, but it is better if it is at least moving, just to be safe.

Thanks to "moose0030" for the tip: If you have the Toy Box DLC, you can use the thrusters in order to have more control over your jump.

"Tarzakul" added this to the above tip: There is no time constraint between your jump and the landing. You could potentially jump from the first car and travel halfway across the city (with thrusters) to the next one, as long as you jump from one car and land on another one, no matter where it is or how long it takes.

As noted in the video, you can do this in co-op, that way you don't have to rely on the inconsistent AI driving skills.

Here is a RoosterTeeth video, in case you want to see it in action (this is in the demo, so the achievement looks kind of weird.):

I tried cars and white box vans for a while without sucess, either due to badly timed landings or the destination vehicle only travelling at normal speed.

Accidentally discovered what I think is probably the easiest (aside from co-op perhaps) method of getting this achievement.

I was at the end of a long straight road (heading onto a bridge) and stood still in front of an approaching car, closely followed by another car. I waited a moment for the second car to pull up behind and then jump onto the first car. Fired a few shots at the second car which immediately accelerated pushing the car I was standing on along in front of it. I then just walked off the first car onto the bonnet of the second and the achievement popped.

Edit: why do people vote negative without leaving a comment to explain why?

The simplist way to get this is if you have live get 2 friends to drive side by side in trucks(i did it with 2 buses) when both trucks are going at a good speed hop from one to the other and your done

I got on the top of the white box van/trucks that are driving around. It happened to pass another one within about 2 mins so just jumped straight onto that one. Unlocked.

Jumping from/to cars will make this one harder to get.

If you still can't seem to get this achievement then use this method.

I simply climbed on top of a civilian car and shot in front of it to get it moving. When i came across a nearby agency car at a standstill while fighting the cell; i simply jumped from my car to the non moving. I got the achievement first time.

This worked for me and a friend. Coop: just park one car on the road and use another car to drive real fast and jump onto the parked car. I think the key is to drive real fast.