I have found there is a huge problem with the amount of trucks on the road.
As far as I can tell trucks can only carry 1 of a resource, and so for each resource created there is another truck on the road. This causes major traffic issues when trying to saturate an area with mines or factories.
Taking concrete as an example, concrete production requires a high amount of input per minute as well as producing a high amount of output per minute. Concrete production creates a massive amount of road traffic between the mines, factories and warehouses. Traffic quickly grinds to a halt as all trucks choose the shortest possible route, even if it is extremely inefficient.
Trains don’t seem to solve this issue as they produce the same amount of traffic once goods have been delivered.
Some possible changes / solutions that would improve trucks overall:
Have a limit to the amount of trucks each factory / mine can operate:
e.g. Each production building has 4 trucks it can use for deliveries. While all trucks are on delivery no more trucks can be sent from the factory.
This incentivizes having production buildings close to the required resource.
You can have upgrades that increase the amount of trucks per factory.
This can tie in well with round trip trucks.
This would reduce the amount of trucks on the roads and improve traffic but would potentially make it more difficult to get resources to habitats on time.
Have bigger versions of production buildings with multiple road connections:
This will allow for incoming and outgoing trucks to be separated onto different roads / sections of roads reducing traffic around important connections at the cost of space.
Have upgrades improving truck carrying capacity:
This would allow trucks to carry more than 1 of a single resource type.
The extra resource could be overstocked in the factory requesting the resource, or sent to another factory requiring the resource after the trucks first delivery.
Improving truck AI:
This would allow trucks to choose another route when they run into a traffic jam.
Have upgraded factories that directly connect to a train line:
This would solve traffic around factory buildings as factories could directly send / receive goods from a train without trucks as a proxy.
Have factory specific delivery routes:
This would allow the player to set specific routes for trucks from a factory / district to follow, overriding the basic truck AI of taking the shortest path to take a slightly longer path with less traffic.
Resource storage / distribution centers:
This would allow trucks to be sent to a warehouse / distribution center that takes resources from production buildings and distributes them to other buildings within a certain range. This would aid in both traffic as well as improve city planning as all buildings would need a distribution center nearby supplying them with their required resource.
These buildings could either stock a single resource or a certain amount of resource types.
Have warehouses (the current ones) with multiple road connections or increased input per minute:
Currently warehouses have a very small amount of available input per minute. (It takes a long time for the next truck to be able to deliver).
Traffic could be improved by allowing more trucks to enter the warehouse per minute or by giving the warehouse more road connections for trucks to use.
Currently trucks will choose the nearest warehouse to deliver to even if there are many other factories delivering to the same warehouse, so some warehouses quickly become overwhelmed while others see no interaction.
Some of the other changes may fix this such as improving truck AI, or allowing factories to have set delivery locations / routes, or having direct train connections, or allowing trucks to carry more resources.