We have used CarTel to monitor urban WiFi connectivity. In a 2006 paper, we report on the results of about 290 total ``drive hours'' from a few cars under typical driving conditions, in and around the Boston metropolitan area. The study was done over a 1-year duration.
500 access points randomly selected from a set of over 13,000 found during the study are shown below.
* Wi-Fi Map (link takes you to a page showing just the map overlay)
Some graphs from this study are shown below. The full paper is here (Mobicom 2006).