Taking this approach to its logical conclusion, one could sample Google to infer a congestion heat map for all the main routes in an area, and calculate the tra

Taking this approach to its logical conclusion, one could sample Google to infer a congestion heat map for all the main routes in an area, and calculate the transit time estimates directly from the inferred heat map. IME congestion is also often modal, there's probably reductions in sampling effort you could make by noticing the patterns in how commuters route, whether school is on break, and what the latest roadworks are.