Lightweight private car routing

[nuhgl]

Good day,

I wanted to start new map of current year 2020.
And build Motorways to see if this can work out financially.

But its seems that my new motorway cannot compete with existing public road.
Most of private car journeys are still done via public road not my motorway.

You can see traffic heat map red on those public road and green on my new motorways.

Is there any advice, I don't understand fully how a private car decide which route it takes.
I don't want to delete existing public road by chaining to public player, since its not realistic.





Especially this intersection is little bit strange,
Grey was public route, and I have built roundabout on it using oneway tool.
Pink route from public road arriving in to the town, there is always traffic but,
Blue routes going to other town form the town, there is no traffic.

[Mariculous]

Your Motorways are rather a detour than a shortcut due to being more curvy than those country roads and those huge roundabouts cause further detours.
Make sure your motorways are a shortcut in a larger scale. Connections in between adjacent towns should not be the job of your motorways, except for rather few cases.

And another hint: roundabouts are fine to handle intersecting bidirectional roads, but usually don't want them on motorways. At least not im Simutrans.
They will bottleneck your motorways in very many cases.
You should prefer motorway junctions instead, although I usually don't build a complete motorway junction connecting all possible directions. I just branch out and merge in before and after towns, so cars can leave the motorway without crossing the opposite direction and the majority of cars can stay  straight on the motorway.

Edit: See the attachment for an example. After the second merge in, there are roughly 2500 cars per month on the motorway (red section)
The orange lines around that are roughly 1200 cars, the green and light yellow lines are rails.

That kind of simple merging and branching is the most common type of motorway exists on that map, although there are also some "usual" exits consisting of one ramp per direction connected to a bidirectional road.
Such exists are fine when there are not many cars leaving the motorway.
If you like roundabouts, you can build a roundabout above or underneath the motorway, connecting the crossing bidirectional roads to it and building ramps to enter or leave the motorway.

[freddyhayward]

If the original roads are congested enough, cars should start using your motorways after a few months have passed. How many have passed in your case, and how congested (i.e. the congestion % in the road tile info) are they?

[Mariculous]

Whilst true in principle, relying on that will cause the motorway to be used unfrequently, as it's only considered as an alternative once the country roads are congested. Once congestion reset, the cycle starts again.

[jamespetts]

Also, how big is your map? If you have a large number of towns connected by road, it can take a long time for the private car routing system to recalculate the routes and therefore for the cars to know about your motorway.

The time that it takes increases exponentially with the number of towns connected by road.