The problem is that from the beginning, TS2 and TS3 had cars; and routing and neighborhoods/worlds were designed for them. Even though TS2 did not have ownable/driveable cars in the base game (added with Freetime or Nightlife), they were still used as carpools and NPC vehicles. TS3 had ownable/driveable cars from the start too.
The amount of work required for cars period would be a truly monumental undertaking. A large amount of coding would need to be done, and worlds/neighborhoods would likely need to be redesigned from scratch. I would doubt any of the routing used for the decorative cars seen in many worlds would do. It was hinted that possibly they were being worked on a year or two ago, and I think that in all honestly, that would likely be the minimum amount of work it would take.
I'm perfectly fine with whatever cars we get. I just want to see them in the game, but I doubt they would happen. I personally think that it's really too late; given cars needed to be in the framework of the game from the start in my view.
