For those of you who can't watch movies at work, just to be clear, you have to drive the Jammer, *edit* sorry the Jetter, the original vehicle Dan Hagar gives you, ram it into a wall and launch yourself through 3 sets of goal posts just off the drivable areas. All are on disk 1.
You won't die, even if you miss, but I do recommend saving at the site of the posts before you try because you'll eventually blow up the ATV if you keep missing.
You also don't want to fling yourself too high, it seems that you have to be between the uprights, and it won't count even if your aim is good if you've gone too fast and flung yourself over the whole thing.
Locations: Just look for the football goal posts just off the map
1. Near the Wasted Garage and your Ark Emergence site
2. Down the long highway by the Mutant Bash TV show, all the way at the end is a building you can't enter, but looks vaguely like it belongs to the authority. the goalpost is to the left of it.
3. In the open space after the Northern Watch Tower, to the right side. This one has a lot of space to launch yourself, but remember it won't count if you are too high. Try to just barely clear the uprights.