Just a few random ideas sparked about the "know your horses" thread.
Problems:
1. A mail bike in modern city cases traffic jams (even if the town is not jammed anyway), or gets caught in jam which in real world would not be a problem for a bike.
2. Game without timeline spawns private horse carriages, causing jams too.
3. It is impossible to build a roadside bay stop, where a bus would pull aside to load/unload, while other traffic can pass in both directions.
Solution.
1. We have a few road vehicles, that are mostly narrow (and slow): bikes, horse riders, livestock drover, maybe others (e.g. pizza delivery motorised scooter). These could be overtaken even if there is traffic in opposite direction. They also could overtake standing (jammed) vehicles. Allowing this would add nice use to mail bike in modern times - priority mail delivery in jammed city... However this would require a new dat option (is_narrow = can_overtake_anywhere = can_be_overtaken_anywhere), similar to flag for double decker buses.
2. The roads in simutrans are quite wide - there is an extra lane in the middle for trams to pass. One tile of rad is 125 m which is quite a lot for overtaking a slow vehicle. Would it be possible to relax the overtaking rules, if the speed of overtaking vehicle is 2x-3x higher than of the one being overtaken? Perhaps with additional check that there are no tram tracks - to make it appear as the overtaking car is using the tram lane. This could apply to overtaking with narrow vehicles too.
3. We have an overtaking mode allowing overtaking "only loading convoi". What would be imho more appropriate is to relax rules for overtaking loading convoy, in similar way as 2. Or to make it a new explicit overtaking mode, perhaps even with some costs (for building the bay).