Congestion

For this post I have decided to spend time talking about everyone’s favorite part of traveling, traffic!

Congestion is never fun. Planners and engineers work constantly to try to mitigate it. Interestingly, supporters of light rail advocate for rail as a means to reduce road congestion. How does that work? Well it all relies on mode shift. Assume we have a constant number of people traveling through a particular corridor, let’s say 100 people. All 100 of these people are driving separately in their own car. If we add a train and 30 of those people now ride the train together instead of driving, then we have decreased the number of cars on the road.

Of course the above scenario does not account for changes in the total number of people. Also, the addition of the rail line more likely than not takes away space from the road that used to be another lane. We could easily have a situation where congestion doesn’t get better, but in fact gets worse. However, the claim light rail advocates state is that congestion will go down.

Let’s see if that’s the case.

There are a couple of ways we can look at congestion. First, we will take a look at the Texas A&M Urban Mobility Scorecard. Every year, this research group puts out a report card on the nation’s congestion. There are national levels and then the scorecard looks at major urban areas.

One measure of congestion is the Travel Time Index (TTI). This takes the the travel time during peak flow (time when most congested) divided by the travel time at free flow speeds (no congestion). Thus a TTI of 1.2 means that a 10 min trip takes 12 min during peak hours.

Below is the TTI for Dallas and LA over time.
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In both of these cities the TTI has gone up (as it has across the country). This matches the overarching narrative that congestion is in fact getting worse in the US.

But these measures look at car travel, let’s focus on how light rail is doing and how it compares with the car.

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These graphs show the mean travel time to work by mode. These data come from the American Community Survey 1-Year estimates. They were calculated using aggregate travel time by mode divided by the number of commuters by mode. The grey bar for transit includes all transit modes (light rail and bus). We see that the travel time to work by transit is nearly double that of car. This is pretty consistent with national numbers. What is particularly interesting is that Dallas saw a noticeable increase in travel time for transit between 2010 and 2011. This increase coincided with an increase in direct route miles. This is surprising because one would suspect that an increase in the amount of light rail service would decrease the travel time, not increase.

Why is that? Well a possible explanation is that the system is now crowded with more trains and people, thus causing congestion. Another idea is that the new extension is bringing on new riders who switched from some other mode (although this idea is harder to explain since that would mean people switched to a slower mode of travel). And possibly, since these number include bus travel as well, there could have been changes in bus service which increased travel time.

While looking at only two cities is not nearly a big enough sample, it does appear that at least for Dallas and Los Angeles, their light rail is not helping with congestion. And for Dallas, light rail may even be making it worse.