It's ever been the case that you need only one survival skill to be able to pass through heavy terrain. So if it's a bug and not a feature, it's a very old one.
Because of this I advocate using "double terrain" on maps only sparingly - but does anyone ever listen to me ?
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As for AI the most effective work around to this problem so far is to simply use an American instead, they tend to put up a bit more of a fight than your average Artificial Idiot. ... James McGuigan on rec.games.computer.stars somewhen back in 1998 ...