There is one lane roads but no one lane bridges or tunnels.
you can turn them one way with the blue icon with the arrow but not reduce number of lanes.

That is correct. The problem remains the two-lane bridges or tunnels. They only make the whole construction project unnecessarily expansive and space-intensive.

to be honest, Highways in this game are unnecessary, if you get the traffic to need them, you will have problems in many other places.
Only reason to build them to any extend is "because i like the look" which is a totaly fine reason, but in of itself already expensive and using space.
An unused lane more or les will not ad much on the global scale of the resources you spend on building a highway.

Okay, understandable.
Let's summarise. I have the option of using a two-lane road as a one-way street. I also have the option of building a second road directly parallel to it and also designating it as a one-way road. I also have the option of building single-lane connecting roads. But no single-lane bridges or tunnels.
Three out of five points for a highway are possible, two are not. And even under socialism, highways were a given.

Cloverleaf interchange is possible with the current system.

Others too, but that requires some terraforming as the one lane sections must pass under the main roads.

you can make a lot of things happen with the one lane road.
i did build clover leaves and trumpets, building service interchanges under or over the highway is both prety doable with one lane roads as well.


the problem is that the road system does not allow to add some things like forks in tunnels and bridges, so that might or might not include one lane bridges or tunnels as well. So i would not expect anything like that ever to be addet.
So we have to work with what we have.

Of course, like me, we have to work with what is available. I just wanted to give some food for thought for the future.
The cloverleaf highway interchange is a fine thing. I haven't ventured into terraforming yet. Instead, I've tried intersections and highway entrances and exits with bridges and roads at ground level.