We had cars in the sims 2 which didn't have open world, and they worked fine; carpools added a nice touch for careers and going to school.
In terms of cars just adding to travel time, think of it like vampires with vampiric run; the travel time is cut slightly shorter because they run quicker to the "exit points" of the lot. Cars would work similarly so it would actually decrease travel time albeit by a small amount.
Besides features don't have to be 100% realistic to add to the game. People complain about the game lacking "depth" but it's the little slightly unnecessary things like cars (even loading screen cars)), cut-scenes, automatic memory systems that made people love the Sims 2 so much, and still now you get people pining for Will Wright's creative input. I don't personally makes sense to compare 4 to 3 because of how different the games are, even if this should not be the case as it is it's successor. The way the Sims 4 works makes it more comparable to Sims 2, and cars were a great addition to Sims 2 loading screens and all.
In short, I'm excited for this. Cars will add more to gameplay than things like laundry, in my view.
