Where is the best place on the map to deploy these units?
Best places would be hexes which are occupied by multiple friendly units and are located in a traffic path. Such places would normally be behind the front line and contain a road. For the MPs to mitigate congestion there needs to be at about 4 or more units (but less than 9) occupying the hex, such as a HQ, several artillery units, an AA unit and anything else you might find supporting the front. MPs assist other units attempting to move through the stack of units thereby reducing the movement penalty.
In my experience using MPs to perform traffic control has been rare. More often they are used clear enemy hexes, chase guerrilla units and dig trenches. Maybe others have actually used them as traffic cops. Hearing their experience would be insightful.
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