In game. Not sure - I only ever see any sort of noticeable congestion near crossings - I plan to go into some of my common maps and strip highway etc from inside many of the towns - the roads are typically narrow and constricted - that might 'help' hinder things a bit...
IRL, most routes would be designated single way, and it would be unwise to attempt to mix columns on the same route - it can also take a long time to push needed units up alongside a stalled column/provide supplies to the lead units on a single route. Much infrastructure and personnel is needed & assigned to managing traffic... Maybe, you could run a foot column and a motorised column at the same time - the foot column keeps to the edges but few roads are sufficiently wide and well graded to allow two-way military vehicular traffic. Because of the bulk of many military vehicles, it is sometimes difficult to even pass a stationary queue of traffic... in a way, I'd say that the road should normally be available to only one moving motorised unit and 'up to' two units of any type (ie 1 mot & 1 foot, 1 mot & 1 stationary mot, or two foot) at any one time.
This per-cell. If the terrain is unsuitable to bypass (eg bridges, bad going) then one unit must halt, both/all replan/re-org and then one may move. The second unit must wait until the first is clear before it can commence movement etc. Where a bypass is possible, then one unit remains on road, halted while the other(s) deploy. Then the first can resume. Once clear, the deployed unit(s) can reform onto the road and move, so long as thhe route is now clear.
One problem with the road/routeing that exists at the moment, is that the rigid road columns for a single unit do not actually follow/lie on the road - the larger the unit, the less likely it is that the head/tail of the column will be anywhere near the nominal route. (Just IMO, but these should be junked, and multi-point/multi-cell road formations that actually trace the road cells would be very useful - perhaps this will be less relevant with platoon sized multi-part formations in the future, but currently it makes it difficult to operate large (mot) formations near any threats, to successfully target the moving formations on roads with air/artillery (only the middle of the unit is necessarily anywhere near the road/target point) etc.
What would be 'useful' is roadblock/crater 'units/tiles' that cause the 'same' movement issues until cleared/bypassed by engineers (or more slowly by any unit).