Just Cause

I think I've found an easier way. Get a car and crash head on into another. Go into stunt position, then just jump from one vehicle to the other and back again. Found this one by accident during a mission.

Find any two lane road with traffic heading in both directions with no centre divide. Any motorway will do. Get a car and enter stunt position driving close to the lanes of oncoming traffic. When a car passes follow the prompt to jump to that vehicle. This car will now speed up but will usually stay in the same lane. When a car comes the other way simply jump onto that vehicle and continue until the achievement unlocks. You can still drive the car from stunt position but the steering is reduced. A straight road should mean that only minor corrections are needed though. If the car you jump to stops or veers offroad then quickly hop through the drivers window and take control driving it back to the centre of the road.

For whatever reason, the game will count jumping between two stationary boats as jumping between car roofs. Once you’re able to call in an Agency boat, borrow any other boat, park it next to the boat, then hop from one to the other and the Achievement is yours.

Also obtainable in the early part of the game during a side mission where you have to go get a camera from a news van. The van is next to a small lake. After the mission is turned in, go back to the lake and take the fishing boat (Trawler) and drive around for a little bit, and another will spawn and then the driver will park it on shore. Go get the other boat, line them up side by side and jump back and forth between the two.