A new London Ring road traffic sign found under Decor / Misc. Decor, costs 0. You need Ambitions and to place the packages in both the mods/packages (Sims3 ) directory and in the "The Sims 3 Create a World Tool/ Packages" directory for the item to show in your game. There is an earlier post explaining how to install items in CAW.
Importing packages into CAW
Alternatively you can place a small lot on the side of the road and place the sign on it.
London Ring road traffic sign (MediaFire)