The way I "do" LODs for making basic AI aircraft is to create a model as I want the user to see it close-up - with all details like aerials etc. I group everything together and call it something like Aircraft_LOD_100.
I clone that model and ungroup everything, and I hide the Aircraft_LOD_100 model so as to clear out the workspace. I then take off all the details that won't be visible at this level of detail (small wing steps, aerials, exhaust pipes etc) and re-group the objects, this time calling them Aircraft_LOD_090.
I clone that model and again ungroup everything, and I hide the Aircraft_LOD_090 model so as to clear out the workspace. I then look for larger parts of the model that I can optimise, such as simplifying a fuselage shape, wheel spats, prop spinner and blades - anything that creates lots of polygons at full detail. When that is done I re-group the objects, this time calling them Aircraft_LOD_050.
I clone that model and again ungroup everything, and I hide the Aircraft_LOD_050 model so as to clear out the workspace. This time, to avoid even more repetition, I'll skip to the "paper plane" part of the LOD model. For this ultra-simple model I draw new boxes or pyramids to take the place of the original aircraft components. Three boxes each the same size as the tailfin, tailplane, mainplane, and a 4 sided pyramid for the fuselage. I stretch the pyramid to be roughly the same height and length as the fuselage. I take off all the sides of these boxes to save a few polygons, and also angle the boxes very slightly to avoid them vanishing entirely if the aircraft is seen head-on. I then remove everything that was in LOD_050 (leaving the "proper" LOD_050 model that we have temporarily hidden), because I was only using those parts to assess the scale of the new boxes I was making for the bottom LOD. When that is done I group the boxes and the pyramid, this time calling them Aircraft_LOD_010. This rather ugly model won't ever be seen closely, because LOD_010 means it only appears at a distance, and costs almost nothing in FPS loss.
I then unhide Aircraft_LOD_100, Aircraft_LOD_090 and Aircraft_LOD_050 and get a messy looking thing on screen that I then export as a single model. In the sim, the LODs switch in according to the numbers I gave them. I assume that the numbers refer to percentages. Call part of a model Aircraft_LOD_090 and I think that version gets displayed when the aircraft is 90% of its full size, and 050 when it is half the size, and 010 when it is at 10%. Something like that, at least. You might need to alter the values for the LODs to get them to switch over at the best moment. For example with my aircraft, if an aerial is about to become invisible anyway (too small for the sim to render it) it is a good moment to switch down to a lower LOD and lose the aerial entirely. I know you're not modelling aircraft, but the same principle applies to other objects such as cargo pallets and vehicles. Even building details, in fact! A friend of mine designed buildings for an airport that used 3D text signage and also rails and pipework, and all of this was LODed so that it vanished the moment it wasn't going to be useful at that detail, either via a flat sign that also eventually disappeared, or just got taken away the moment it wasn't worth rendering. The basic buildings remained (most are simple boxes anyway, not really worth LODing).
If something like your taxiway lines are a frame-rate problem you'd probably have to build them in sections (eg all the lines in a particular part of the airport) and possibly have 2 LOD models per area (eg terminal area, cargo area and military area, for example - if they are all far apart you can't see the lines over in the cargo area from the terminal area, but the sim will still try to draw them). One LOD would contain all the lines for an area and there'd be a dummy LOD containing none of them. That way, the lines would get "switched off" beyond a certain distance from the centre of the model. How you sectioned out the lines to prevent them visibly appearing or disappearing would depend largely upon the airport configuration. That's just my thoughts, at least.
