Update 2011-11-12 8:21 -0800: I just posted a visualization I like better. The Internet has been abuzz the past week regarding transit speed maps. It seems to have been spurred by a post on Bostongraphy, which was inspired by many of the amazing visualizations produced by Eric Fischer, especially this one. Indeed, this blog has gotten a fair bit of traffic itself, because Andy Woodruff of Bostonography used my avl2postgis project to retrieve the data.

Some readers may have noticed that I’ve updated my last post several times in the last few days. After thinking about the algorithms I used, I realized there were some significant issues with them. I’ve explained them a certain amount in my updates to my previous post, but I’d like to expand on the issues a bit here. Using an Inverse Distance Weighting algorithm exaggerates delays where stops are sparse by allowing them to spread over larger areas; the graphic should make this clear; if the red dots are stops with delays, one in the city center and one in a suburb, it is clear that the delays will be magnified where stops are sparse (figure 1), because there are less stops around it.