You probably have a good reason for setposing the Bmp to the road at a certain point in your cutscene...but if not, this may work.
I've noticed the drivers always suck when they first start out, but once they get moving along the road (in careless mode), they prefer the road, and will stick to it. So if you can start the BMP out of town a bit on the road, and give him enough time to start driving well, he can arrive at the desired point at the desired time (depending on starting placement).
Also, to build on mac's point, any units near the road will disturb the AI driving.
And finally, I helped Cheetah fix a convoy driving problem in his Falling Stars mission by setting dammage to 1 to all static objects near the road (trees, streetlamps, bushes, signs, etc.). This worked pretty well:
"object _x setdamage 1" ForEach [14595, 14596, 14588, 14644, 14645, etc.......]
You find all the object ids of object near the road, and make a trigger that is true once with the above line of code (using your object ids)