Setting <is_road_tile=true> to the first function it only enables for basic tiles.

This does not work for rotated tiles and markings. Just the first ones.

There are another functions where it needs to be placed starting from line 172, 179, 184 up to line 236.
Somewhere in here needs to be placed this line as well.
But I have not found an easy solution to this.

Also an "bug" in transport drones still exist that the drones could cross non road tile if the other side is connected to the same network.
They only cross one such block if it is shortcut.

You can test this with 3 block wide road with depots positioned directly opposite to each other at the center line.
If you make "road block" with non road tile, in 1 and 2 line and leave 3 road tile. The drones willl cross the at the center line 2.
Making "road block" 2 tile wide will make the drones to avoid it.

So in our situation if you place road markings here and there the road network may work and appear all fine.
But if you overdraw it fully with arrows left, right the network may stop working.

Thanks - I have no idea how I didn't notice this in testing! I will see how many of these snags I can work through and let you know.

Sorry for the lag on this. Real life intervened. I tracked this down to an issue within the transport drones mod. With some help from Klonan, I've submitted a fix. So next transport drones release should fix this for you.

By "this" I mean the issues with rotated tiles not being picked up as road.

I have instaled new version of Transport Drones (1.0.11) and tested it again.

I can confirm the issue is fixed from Transport Drones mod.

Also tested overdrawing suply depot with markings right in front of it. This seems to work as well.

This fix will help also other peoples that want to play with city blocks with roads.
This mod was downloaded quite few times since these 2 months.
Not just me.

Thank you and Klonan for a fix.

Welcome :D Now that the really hard bits have been figured out, I'm happy to add support for other modpack-specific road surfaces as they are requested. Enjoy! And if you make any really pretty cities then please post pictures :)