It is possible to run the game without the .ZAD files, using all assets extracted, so that you can install mods and modify the files yourself and run them as is, instead of repacking everything into the .ZAD files.
The game ends up taking 15gb instead of 6...
To run the game extracted you have to:
1. Extract all .ZAD files from the ZAD folder somewhere, and replace any files that is asked to replace (the only files that will be replaced are the ones in the .METADATA folder, those will not be used)
2. Delete the .METADATA folder that was just extracted.
3. Move the Data_Core folder to the main Carmageddon folder (\SteamApps\common\Carmageddon_Reincarnation)
4. Move the ZAD folder somewhere else, so that the game loads from the folder instead of the ZAD files.
Ps: Keep the ZAD files saved in another folder, this way when theres a new update you can move them back to the right place and update, instead of redownloading everything.