How does route calculation work?

Traffic generally just takes the shortest route which can easily cause congestion.

There are several ways to combat it in my experience:

• Use public transport (obviously that won't help your truck traffic though)
• Designing of the roads so that trucks for example will have to go to a larger road, almost like having separate areas of you city only connected by highways
• Use policies to restrict traffic, there is one for no HGVs
• Using the Traffic Manager: President Edition mod so you can restrict traffic types on specific roads. This will give you so much power over almost all parts of your transport system, I wouldn't play without it! Traffic restrictions, junction restrictions and lane joining make funneling your traffic around so much easier