Well, you decide where put them, isn't this the big problem if you know how to add them in the in-game cities.I could do it, but I wouldn't know where to put some of the props.
About the signals, like the ones of the obligation to turn to a direction and of the no entry, you have to place them following the London traffic directions, and it's pretty hard.
There's a lot to do to let the player car go there. Good luck.However, I think I've found a way to work around them. That's why I said maybe.