"Signalized intersections" are an odd combination of engineering and politics.
Target stores, for instance, has a history of paying municipalities to put up traffic lights in front of their stores.
Apparently it has as much to do with marketing theory as it does traffic flow, but Im guessing that municipalities take into account that they dont pay the whole installation cost and figure "why not?".
Way off topic, but I was in Minnesota last week, and noticed the small towns usually have a Target instead of a Wal Mart and Wal Marts tend to be in the larger towns. Quite the opposite of OK/AR/MO/KS. The reason for this is obvious, but I still found it interesting.