A little plea to those less experienced, and also to those experts who know everything already.
Add a Roadway LOD to your add-on. Not only (as was discovered by the Founding Fathers of OFP modmaking) does it mean surfaces become walkable, but they can also become jumpable and sticky-when-moving.
I have no idea if BIS are going to add these latter two features, and I'm sure that if they did it would be one hell of a lot better than my upcoming new-fangled half-arsed way of doing it. But regardless, if you want some cool stuff to happen with your add-on, be sure to add a Roadway LOD. Include in it ANY surface, be it on a car, a plane or a building that you suspect someone will want to walk on & off / jump on & off / "stick to when moving".
By the way, Roadway LODs are very simple creatures to make. Have a look at the Roadway LOD in an object like the very nice LSD someone released recently. (Sorry, forget the name of the maker, but I'm sure everyone knows the model).
If you're making a jeep, for example, you'd make the roof surface, the bonnet surface and the windshield surface parts of your Roadway LOD.
Edit: By the way, you don't need to bother about a Pathway LOD for this, just a Roadway LOD.
Yeah I always add roadway to may LOD - it annoys me on some buildings like Res. Warehouse/hangers which haave no decent roadway on roof.
To stop stickyness and being shaken about when u walk on roadway, I always find this stops if your roadway surface is slighly higher than the other LOD - this means u are elevated slighly above geomotry collision model which stops this happening.
I should also mention the importance of the Geometry LOD in this regard.
You need to have a proper ceiling defined in the geometry LOD so that you can't go up through it by jumping. If this happens, you end up on the floor above - not good. Again this is simple to do, and the ceiling does not have to be hugely thick. I will post a ballpark figure for thickness when I've finished testing (although I am currently playing around with ceilings between 0.25 and 0.50 meters thick).