How Do We Quantify Traffic Congestion?

Poorly, at least according to this article: http://dc.streetsblog.org/2012/08/15/theres-a-lot-riding-on-u-s-dots-definition-of-congestion/

Congress recently passed a very highway (and sprawl) friendly piece of legislation known as MAP-21, and it seems likely that they will use the current ineffective and outdated method of quantifying traffic performance and congestion.

The current way of measuring traffic performance is heavily biased towards what percentage of the average trip occurs in un-congested traffic, which tends to promote sprawl. The further away someone is forced to live from where they need to go for work, school, necessities, etc., the more likely it is that less of their drive time will be in heavy traffic, even if their overall commute time would be significantly shorter, albeit potentially in heavy traffic, if they were closer. Consider this graph from the article:

The city of Charlotte scores better than Chicago using the current index, even though distance and time-wise it takes longer to get where you need to go, simply because a smaller percentage of the drivers’ time is spent in heavy traffic. Isn’t this counter-intuitive? It seems more logical to give overall travel time more importance. The goal should be to spend less time in our cars, not a smaller percentage of time in traffic. We experience time incrementally, so every minute spent in a car, whether in traffic or not, is a minute not spent on something more productive. I don’t know about you, but I have a hard time breaking down my day into percentages and really understanding what it is I’ve gained or lost. It tells me nothing to know that I spend 38% of my commute to work in heavy traffic, but it tells me everything I need to know to say that it took me 18 minutes to get there. I’ll take a shorter overall commute time over a shorter percentage of time spent in traffic any day.

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