I posed the question to Bill Scott. He is the 2003 WBC PZB Champion and the 2004 WBC PZB GameMaster. Here is Bill's response:

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In my judgement, for PB, stacking limits do not matter during movement phase - only before and after - as per rules instructions (only as regards road movement do limits matter and of course that means primarily moving along roads and thru swamp hexes and thru green hex sides of woods - which is very confusing and even I have to continually reference the rules which David Giordano and Alan Arvold basically decided and Alan explains pretty well in the "Boardgamers" guide to PB rules. If I had my way I'd make the PB road movement rules more consistant with common sense but I hate to change tradition ....... If units are stacked together on a road hex at the beginning of a turn's movement phase they can all move at the road rate to start their move including thru a green hex side or along a road/swamp. But if not starting together, or after the 1st hex of movement they must move at the non road rate if they are stacked (even for only 1 hex of movement). This rule should always be clarified between players at the start of a game since I estimate that 90% + players will discover they disagree with how to play the rule. When I run a tournament I try always to clarify this crazy rule with all players and I remind everyone to make sure they are in agreement. Its even harder for players familiar with both PB & PL rules since they are different and its so easy to find oneself playing by the wrong rule.
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Tom
I play it as not considering stacking during movement. The one exception might be not allowing road movement through a green hexside when a piece is parked in the hex to be entered. It can be argued the parked piece will be moved away later, which would in theory clear the road. But, the blocking piece may have already moved, or it might not be moved that Movement Phase.