Road congestion and travel

[Zoetermeer]

I can't quite figure out how the presence of other units on a road affects movement costs for traversing that road.  For example, suppose I have three motorized units all in a stack in travel mode, on a highway.  If I select all three and ask them to move to a spot N hexes away, they will all go toward it but won't be able to go as far as they would had I selected each one and moved them individually.  How can I quickly and efficiently move whole stacks along roads in this manner?

Do roads have state that represents the number of units having traversed them in the current turn, to simulate the fact that everything within a turn is simultaneous movement?