Ok, remember how I mentioned I figured out how to apply surface properties to roadways in ArmA?
Well, that's actually in the Oxygen2 modelling package that the game developers have released. But anyway, I mentioned there that "dirt" was the default surface property. That was because when I walked/drove on the bridge, it behaved like dirt (crunching underfoot, dust clouds from the vehicle wheels, etc). Turns out that's because I drove/walked on some dirt before I got on to the bridge approach ramp in my test mission.
The default action is to continue applying the last surface properties used. If you move from dirt onto an undefined surface, you get dirt properties. If you move from metal decking to undefined, you get metal properties. The obvious solution is to apply properties to all your roadway LOD sections. And now I know how to do that, all is good on the surface properties front.
Shame I can't get ladders & lights to work... :-(