To make motorcycles, sports cars, classic cars, BDF trucks added by other mods use the special rules from RTD, you will need to rename a file inside these other mods.

Remember that you will need to rename the file every time you update, replace, or add a new traffic mod that adds these traffic types.

Open the traffic pack .scs file from your Documents\Euro Truck Simulator 2\mod folder, or the Steam Workshop file
C:\Program Files (x86)\Steam\steamapps\workshop\content\227300\[id of the mod]

An .scs file is a renamed .zip file so you can use Winzip, 7-Zip, or another archiver for this. In the case of a Steam Workshop mod the mod will either be loose files in folders, or it will be inside of a .zip archive.

Update June 2024: recently, the new HashFS v2 format for mods was released, which means .scs files cannot all be opened by a Zip client any longer. For those files you will now need to use a HashFS v2 extractor to unpack the entire file into a folder. Use SCS Packer ( https://modding.scssoft.com/wiki/Documentation/Tools/Game_Archive_Packer ) or the extractor here: https://github.com/sk-zk/Extractor
Then follow the rest of the steps.

Inside the archive or Workshop folder go to \def\vehicle\
There rename the traffic_storage_type.identifier.sii or .sui filename,
where type is car or truck and identifier is the mod internal name.

Leave the identifier, e.g. jazzycat_moto or classic_tm, as is and also do not replace the .sii or .sui extension.

Only edit car or truck types.
Never edit any other traffic_storage files like those that begin with traffic_storage_car_city, traffic_storage_car_custom, traffic_storage_bus, traffic_storage_police, or traffic_storage_trailer_truck.

Motorcycles are any kind of two-wheeled traffic. An example is Jazzycat's motorcyle traffic.

In the case of Jazzycat's motor traffic, open motorcycle_traffic_pack_by_Jazzycat_version.scs and go to the \def\vehicle\ folder, then rename the file to this:
traffic_storage_moto.jazzycat_moto.sii

BDFs, box trucks, or lorries are traffic trucks where the cargo is part of the vehicle, not articulated trucks. An example of a mod which adds this traffic is Jazzycat's Painted BDF Traffic Pack

In the case of Jazzycat's mod, open painted_bdf_traffic_pack_by_Jazzycat_version.scs and go to the \def\vehicle\ folder, then rename the files to this:
traffic_storage_truck_bdf.jazzycat_bdf.sii


Sports cars are vehicles like Lamborghinis and Ferraris, racing cars. These will want to drive at or above the speed limit in nearly all cases.
TrafficManiac has mods for this for ETS2.

Make sure the entries under \def\vehicle have the name traffic_storage_sport such as this: traffic_storage_sport.tm.sii

Classic cars are old timers. They will drive below the speed limit a lot of times and less aggressively.
TrafficManiac has a pack for this but you could also for example set Jazzycat's Russian Traffic to this.

classic_cars_traffic_pack_by_TrafficManiac_version_base.scs
russian_traffic_pack_by_Jazzycat_version_base.scs

Make sure the entries under \def\vehicle have the name traffic_storage_classic such as this: traffic_storage_classic.classic_tm.sii
traffic_storage_classic.jazzycat_rus_traffic.sii


Assuming you've already edited the base traffic packs as per the above instructions, to also get them to appear on parking spots you may need to edit the actual vehicle files. Not all mod authors disallow parked vehicles, but for example Jazzycat and Trafficmaniac do tend to block vehicles by default.

Open the traffic pack base .scs or .zip file and extract the entire \def\vehicle\ai folder somewhere.
Using Notepad++ (or another text editor that can find and replace across multiple files at once), open one of the .sui files under \ai\ or a subfolder of it.

Press ctrl+h to open the Replace dialog and go to the 'Find in Files' tab.
Find what: allow_parked: false
Replace with: allow_parked: true
Filters: *.sui

Select 'Follow current doc.' and 'In all sub-folders'.

Click 'Replace in Files' and let it finish. Once done, close Notepad++ and put the entire \ai\ folder back in the .scs or .zip file, overwriting if asked.
If the mod was a HashFS v2 file, use SCS Packer to repack, or alternatively use a ZIP client to re-zip the entire folder under extracted.and rename it back to the original filename with the .scs extension.

You will need to repeat this step every time you download a new mod or an update.