I made something similar to this in a mission I am/was working on. The mission has you sitting in the gunner's seat of a PBR boat, which is driving thru a river (in Dak Seang island).
The boat is supposed to haul @$$ down the river, with you shooting wildly at the enemies on both banks, while the enemys are trying to shoot at you. Problem is, the AI driver doesn't want to go very fast, like it would if you were in the open ocean. So I made a little script that forces him to keep driving fast:
;speeds up a boat, since the AI drivers don't want to go very fast
_boat = _this select 0
#loop
? speed _boat < MAX_SPEED : _boat setvelocity [(velocity _boat select 0)*1.1, (velocity _boat select 1)*1.1, (velocity _boat select 2)]
~1
? alive _boat && alive driver _boat : goto "loop"
exit
This script basically makes the boat constantly accelerate in the direction it is facing, until it hits the top speed (set in a global variable so I can adjust it when the boats reach certain points). The driver is still able to turn the boat towards his waypoints, and the whole thing usually ends up working very well.
The path of the boats in my mission is generally pretty straight though. There are only a few sharp turns, before which I lower the speed of the boats. If you want the jeeps to make 90* turns around building corners and stuff, this might not work. But hey, it's a start.