Thank you for your helpful advices! I want to list up issues now I have.
1. Simutrans was updated to 120.2. The current patch cannot be applied to the new version of simutrans standard since the structure of files changed. Although it seems that there is no relations between new features of 120.2 and this extension, codes themselves seems to have been changed because of changes from German to English. So, I cannot simply replaces files. Since generally simutrans version 12X.X.'0' have severe bugs in many cases, I want to wait until the latest stable version is confirmed as a really stable version. Then, I will move the base of this extension to the latest release, but it must take a long time.
Cars on the left lane enter the right roads crossing over vehicles.
This is a bug and should be fixed. Vehicles entering road of the opposite side should check before crossing lane. I'm reading the code to search how to fix it.
In fact, vehicles entering right roads should go back to the right lane before changing their direction and this should be guaranteed when the vehicles start overtaking. However, in this patch, vehicles do not know when they can go back to the right lane and to know this is impossible. So, I think the best way to prevent this collision is to check before crossing lane.
It appears to be a lot more difficult for a vehicle to initiate overtaking of a City car (especially a slow City car) than a player vehicle. Is there something in the way City cars are coded that would make them more difficult to overtake?
In fact, conditions to overtake city cars is not changed yet (still isidoro's condition) ! The new conditions for city cars should be written and I'm planning to do that, sorry.
Overtaking of stationary vehicles at a stop has improved a lot, but sometimes a queue of stationary traffic still forms behind a stopped vehicle.
The cause of this is still in research. Maybe one of the causes is the condition of "safe space". This condition makes vehicles confirm that the opposite lane is empty for at least one tile in front and in rear when changing lane, but I think there is another and bigger cause of this problem.
5. In my editing code, overtaking vehicles which have larger power is now prohibited. (same power is allowed.) This will reduce the blocking by slow vehicles without ruining the aim of this extension.
One-way Two-lane road fun patch (OTRP) is still on development and there are so many things to be done so that this extension is integrated to simutrans standard or extended. I will continue writing the code until this patch is completed.