Wednesday, June 19, 2013

What’s Wrong with Estimating Traffic Congestion Using GPS Statistics?

The problems with measuring traffic congestion (Felix Salmon, Reuters, Oct. 17, 2012  

Also discussed here: The problems of measuring traffic congestion (Road Pricing, May 15, 2013)

And here: TomTom Congestion Index(Tom Tom, Mar. 22, 2013) And here: TomTom North American Congestion Index(68 page pdf, Tom Tom, Mar. 22, 2013)

Today we review an article that criticizes the effort to estimate congestion in 120 cities world-wide by a major GPS producer, TomTom. The author, Felix Salmon, points out several issues that reduce confidence in the city by city congestion indices: the lack of an independent measure of congestion that would calibrate the GPS-based estimates, the assumption that the speeds of vehicles using the GPS represent the speeds of the average or all vehicles on the road at the time and the weight given to various road segments being measured. The bottom line is that the efforts to reduce congestion by road pricing require a sound way of measuring congestion in order to recognize success or failure of the pricing method used in a given city. While TomTom should be congratulated for its efforts so far, it seems that a second or third objective and consistent way of measuring congestion is needed.

 tom tom congestion  

Key Quotes:

 “A congestion index like this one serves two purposes.
  • to compare a city to itself, over time: is congestion getting better, or is it getting worse?
  • to compare cities to each other: is congestion worse in Washington than it is in Boston?”
“TomTom only looks at road segments where they have a large sample size of traffic speeds — big enough to give “statistically sound results.. The next step is weighting the different road segments, giving most weight to the most-travelled bits of road…But if you want to work out where a city’s drivers drive the most and drive the least, then you need a much larger and much more representative sample….TomTom is always going to get data weighted heavily towards people who don’t know where they’re going — out-of-towners, or drivers headed to unfamiliar destinations.”

“measuring congestion on a percentage basis tends to make smaller, denser cities seem worse than they actually are”

 “the whole idea of congestion charging has a very big problem at its core. There’s no point in implementing a congestion charge unless you think it’s going to do some good — unless, that is, you think that it’s going to decrease congestion”
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