Recent time attack demos and videos have featured lots of weird tricks for Half-Life 2, but this is one of the easiest, funniest and perhaps most useful.
First drive the buggy next to this cliff. Do not drive it in the garage.
Then you have to get some health and a grenade for your MP7. Both can be found from this building, but if you're low on health, you should consider looting the lighthouse also.
Next shoot the buggy up the cliff with the gravity gun. Shooting cars around can take a while to master, but you can try the Car shoving time attack for practice.
Pile up some stuff so you can climb over the small rock in the center.
Once you're up, aim down and press the secondary trigger of your MP7 and jump at the same time. This technique is called grenade jumping, and you can apply it to many situations throughout the game. You should also crouch in midair so you can reach the edge of the cliff easier. It might take a couple of tries, but it's worth of it :)
Once you're up, just drive the buggy down this cliff. If you want to do it without the buggy, you can slide along the cliff on your left. You'll still take heavy falling damage, but it's possible. Next drive/walk back a couple of meters, so you'll reach the next level trigger.
This part of Sandtraps wasn't really designed for the buggy, so driving forward might be a bit troublesome. I usually dump the buggy right after I've reached the rocky area, but you can drive & gravity gun it all the way to the gates of Nova Prospekt if you really want to.