I had a foray in to map making, and the best way I can describe it is that it is a Core part of the Game-Engine.
Some one may be able to explain better, but essentially roads & routing can be a nightmare. The modifiers only cause the units to slow down over hedges, fences & thru marsh & water etc. If you adjust the modifier below a certain number for roads for example, the roads cease to function or be recognized as roads. It's a game engine limitation I think.
Since you wanted faster troop movement I figured your best bet was to return the modifiers to their original settings and increase UnitPerYard in the maps.ini file, or use the turbo mod.
I'm just trying to help you get a desired result using known work arounds. Adjusting the wrong modifier will un-happily just break things.
Furthermore, where roads are concerned I can tell you that I had a heck of a time getting mine to work. For the life of me they would not use the center of the road and for some reason found the grass verge the faster surface to walk on (to give you some indication of the headache it is to script)