This is a simple traffic loading fix, for people who have modded cars and get the issue of always the same car appearing. It is also a welcome addition for people who wish to have more diverse traffic.
You will need an asi loader and aru's Scripthook, or something similar. Be sure you GTA IV can load .asi scripts. This script has been tested on 22.214.171.124 (with ScriptHook 0.4.0), 126.96.36.199 (ScriptHook 0.4.0) and 188.8.131.52 (with ScriptHook 0.5.1).
To install this mod, copy the files TrafficLoad.asi and TrafficLoad.ini in your GTA IV folder. Remove all older or other versions!
You can specify your own values for both pedestrians and cars in the .ini. It is also possible to modify the amount of cars loaded before models that aren't used anymore are wiped from memory.
When you have no added cars installed, no modification is needed. If you do have added cars, look in the .ini for clarification. Up to 15 cars can be added.
If the mod does not load, check ScriptHook.log for possible messages, or the lack thereof. If nothing is loaded, be sure to right click the file and check for any security measurements Windows Vista and Windows 7 apply. Remove those, they prevent the file from getting loaded.
When it still doesn't load or it loads but a weird memory address pops up in the logs, copy and paste MSVCR100D.dll into your GTA IV folder. (Thanks to BehelitOutlaw for finding that)
Or download it here: http://www.dll-files.com/dllindex/dll-files.shtml?msvcr100d
If you experience crashes and/or texture loading trouble, you might try to lower the amount of cars per cycle.
How this mod works and further things you maybe would like to know
This fix randomly loads any car into the memory. After [the amount you specified] car models are loaded it tells the game to mark ALL cars as unneeded if they exist. This causes the game to unload any cars which aren't on screen anymore. After that, it randomly loads 26 cars again. This process repeats the entire time the game is loaded. Each cycle takes roughly [the amount you specified] seconds, but you won't notice anything. Just give the script some time to load, [the amount you specified]*2.5 seconds after loading a savegame, traffic diversity is constant.
Busses and taxis however do not participate in the cycle. They always load no matter what has been chosen, they just load. Busses appear regulary and all taxi variants appear in a good mixture.
This way of loading offers far more diversity than older methods. Cars which aren't loaded normally appear in traffic. Sports cars load sparingly, but they all load. Even the Infernus and Comet load. You won't notice repeating cycles anymore consisting of 30 different cars, all 84 cars (excluding service cars and taxis) are loaded. Bikes also appear normally since 03-10-2011.