Why has the UHI effect stopped for 15+ years?

The Urban Heat Island (UHI) effect.
According to Guru Watts the UHI accounts for a large part of the temperature increase seen since the 1970s ...
Using WUWT as a resource of facts!

(update 2013/10/22 - To find the posts just do a search on watts site.)

Climate Craziness of the week – with the physical signature of UHI staring them right in the face, Mann & Borenstein go with their ‘gut’ instincts