Traffic belongs to any cars on the road, no particular model. I'm guessing the number refers to subsections of cars, certain dialog will be applicable to certain traffic while others will not. Replacing a playable character is simple, but has a few steps to it:
Go into the "leveli.mfk" file in the scripts/missions folder and find this line of code:
AddCharacter("homer","homer");
Change both sides to the name of the character you want. If the character you want isn't already a playable character (ie. not Homer, Bart, Lisa, Marge, or Apu), there's a couple of other things you need to do.
You can't create animations, so what you're looking for is animations already found in the playable characters that you think is applicable to yours. Say, if I think playing as Barney would be cool. I'd probably use Homer's idle animations.
I'd take the "homer_a" file in the art/chars folder, copy it, and rename it "barney_a". I'd take the "homer.cho" file in the same folder, go into it and rename any instance of "homer" to "barney". Make sure not to rename the animation names. Save that as "barney.cho".
Copy "homer_kickwave" and rename that to "barney_kickwave". Now the "homer_electrocuted" is up to you if you want to change it, if you want the electrocuted texture to look more like your character, you'll have to edit the image found in the .p3d file manually. Save it, and of course, rename it to "barney_electrocuted".
Go into the "level.mfk" file (not leveli.mfk) in the scripts\missions folder and find these lines:
LoadP3DFile("art\chars\homer_kickwave.p3d");
LoadP3DFile("art\chars\homer_electrocuted.p3d");
Rename those appropriately. Essentially, you now have a functioning playable character. The next step would be adding dialog options for your character, for example, when Bart says "my ovaries" while running. It's a lot more complicated, but I'm happy to run through the steps with you.
Unfortunately, I'm about to get onto a plane. So it might be a while.