I hate to be a blatant self-promoter, but, well..... here I go ;D
I think macguba was right on with his last suggestion. A group will only move as fast as its leader; therefore if you put a man on foot leading the UAZ's, the cars should only go as fast as the leader. Don't put people in "careless"; they will basically ignore enemy fire if you do that. Use "safe" for the vehicles (they don't like to drive on the roads otherwise), but use "aware" for the men on foot, or else I think they will shoulder their weapons (which will slow the convoy WAY down...).
Okay, now for the self-promotion. Maybe you could try using my Custom Formations Script Pack (r) to make a custom formation for your convoy. It may work pretty well. You can get it through the link in my signature. A readme is included, but you can email me if you have any questions. Secondly, grab my mission platoon commander (also linked in my signature, or you can get it in the missions depot). It is an ambush mission like yours. And I started making the mission not knowing much about scripting at all. I gradually learned more and more as I made it, but all in all it doesn't have a whole lot of scripting in it. Un-PBO it and have a look. Again, email me with any questions, if you wish. Oh, and in case you've never un-pbo'd anything, you can get a program to do it here:
http://www.ofpec.com/editors/resource_view.php?id=358 It's self-explainatory how to use it.
Now, a note on realism: unless the enemy thinks it is in COMPLETE safety, he should be weary of attack. Now, I don't see why there would be men running on foot, instead of riding in trucks/cars, if they thought they were in complete safety. Hence, maybe you should give the enemy a chance, instead of lining him up for a picture-perfect ambush. An easy tactic is to have men on the flanks of the convoy. Basically, they would be maybe 50 meters (depending on terrain) away from the road on each side or the convoy, and their job is to basically check for any ambushes. I did this in my mission, and it sure makes an unpleasant surprise for the ambushers. Also you should have your scouts much further in front (as far away as they can be and still see the convoy). If you really want to get into it, you could have the soviets call for artillery support, but now we're getting complicated
Hope this helps!