Roads should have a maximum capacity.
As it is now, a dirt road can move 1,000 logistic points or 100,000 logistic points. The LPs are only limited by the number of trucks created in truck stations and their action points. However, roads do not have unlimited space.
The player only builds a road once, so there is no mechanism to expand the road into more lanes.
Sealed roads are helpful. However, the impact is more focused on throughput. Throughput is a reasonable abstraction--trucks proceed quicker given better surface and on ostensibly more lanes (like a highway/freeway/autobahn). However, there is no limit. 1,000 or 100,000 logistic points can be moved along the same sealed road in a turn.
Finally, no consideration is taken for other assets moving on the road, like units using their own action points. A unit conducting a road march on a road is going to impact the capacity and throughput of other users on the road.
Unit movement should impact the preview points as a pay it forward mechanism in terms of the road's capacity.
If the road's capacity is exceeded by the unit's movement, the road's benefit to unit movement should be removed for further movement--the unit has to travel cross country and not on the hex's road.
much of the same is true for rail in terms of logistic points. limited main lines.
Population & worker wages also a limitations too, so you never see even millions of logistic points without extreme cheating.