Thank you for your thoughts.
Wouldn't it be better to draw an icon on top?
Draw nothing in case of "Two way (default)" is fine.
Drawing the arrows in case of oneway is fine either, but might better be the oneway sign.
As already pointed out, it is the same as the existing arrow design is bad.
I think that one of the factors that should be considered is the visibility when zoomed out.
The three-dimensional like drawing reduces the visibility during zoom out. Simple image is better than complex picture.

no overtaking - show solid centre line
overtaking only stopped vehicles - show dashed or dotted centre line
opposite lane, show red arrows in respective lane and direction.
dead end, show colored line at the tile border opposite the tile's exit
I think this kind of simple expression is better, but even then, the visibility when zoomed out may not be good and I think it still requires some work.
Coloring tiles is a minimal change that uses existing code. There are many priority issues that must be resolved for this feature...
Anyway, improved visibility will make it easier to find errors.
The colors are easy to see, but these are comletely unintuitive.
Do you think the color of block reservation is intuitive? I think it's the same. Also, the idea of coloring tiles may be consistent throughout the game.
About color
1) Color that does not conflict with Block reservation
2) the constraint should be near red, otherwise blue or green
To judge from the image:
Yellow-green is similar to the station color and may have poor visibility.
Pink may be slightly hard to see. Could a darker color be better?
Please note that these two are basically used only for bus stops.
I don't know what inverted is for.
Bus stop restrictions are only required for bus stops. There is no reason to set this to anything other than a bus stop. It's just confusing. And, as explained above, stationary only means no overtaking on tiles other than bus stops. I think a lot of work is required to change this, such as rewriting set_overtaking_mode above.
Regarding the drawing of arrows, there is currently no prospect of what to do. However, as a temporary change, I think that measures such as making the shape simple and changing the color may be possible. It should be considered in conjunction with the tile constraint colors.
We already use that for reservations and player ownership.

The problem is not so different in any way, and I think that tile coloring may be the best.
Finally I hope you understand that this is a simple fix. I'm not familiar with these codes so I may not be able to do any further.
I wondered if I could color the tiles with, for example, "no entry sign" or "mothbold", but it seemed complicated. Coloring in overtaking mode was easy.