Ways of measuring D.C. traffic congestion need to be as varied as our commutes

Robert Thomson
Columnist February 20, 2013

Dear Dr. Gridlock:

You asked us how best to measure congestion [Dr. Gridlock, Feb. 10]. The old Texas Transportation Institute way of using trip time is accurate, but not meaningful. Measuring trip time penalizes a larger urban area and favors a smaller one.

Robert Thomson is The Washington Post’s “Dr. Gridlock.” He answers travelers’ questions, listens to their complaints and shares their pain on the roads, trains and buses in the Washington region. View Archive

A shorter distance takes less time to travel than a longer one. The national capital region, from Loudoun County to Prince George’s County and from Montgomery County to Prince William County, just takes a long time to cover even without much congestion.

To get cheaper houses, people move to Prince George’s and Prince William, which adds time to their commute but might not measure congestion.

You mentioned a new measure of perceived delay. This has potential, but perception might deviate from reality and depend on different expectations. It might be far from statistically reliable to compare one area with another.

A third method is developing to measure speed of commuting in miles per hour. This is done by Global Positioning System units on participating trucks, so it is highly accurate and meaningful. Forty-five minutes is thought to be the maximum tolerance for the average person’s commute. The TTI finds many commutes around 33 minutes.

Under the GPS unit measure, Raleigh, N.C., is one of the most congested cities, and we are 14th.

I am beginning to think we need to use all three measures so that a resident can pick which problem is most important to to him: speed, time or distance.

Ed Tennyson, Vienna

DG: Tennyson identifies the key issue with national measures of mobility, such as the one developed by the Texas Transportation Institute. The institute does the nationally known study that, with depressing regularity, shows the Washington region as having the worst traffic among large urban areas.

No one formula or index reflects all the complexities that make up urban commutes. Some travelers long for quicker trips, others want a consistent travel time and some would like to shorten the distance. They can control some elements and not others.

What makes one person’s commute miserable might not stress another traveler in another part of the region. So I’ve been asking readers to describe their personal pain indices.

But, like Tennyson — a consulting engineer and former deputy secretary of transportation for Pennsylvania — I do think national measures have something to tell us. You can look at a variety of them and try to find patterns over the years.

The institute’s researchers continue to refine their work. Their calculations on congestion now benefit from traffic speed data provided by Inrix, a company that collects travel time information.

Many of you who look at online traffic maps or at traffic apps on smartphones are seeing traffic speed data provided by Inrix. The company gathers information from GPS units in vehicles and from mobile devices.

The number of indices in the institute’s reports is expanding. As Tennyson noted, I was particularly taken with a new one called the Planning Time Index, which measures the extra time that urban travelers need to build into their schedules to ensure they are late no more than once in 20 trips.

They’re not telling you when to leave for the airport. They’re trying to build a benchmark index to compare travel-time reliability among urban areas from year to year.

So far, though, no one has found a mobility index that makes us look good. Although your experience varies, the studies are telling us that nothing we’ve done in recent years — no highway plan or transit plan — is making our collective experience any better.

EYEs up at stoplights

Dear Dr. Gridlock:

The good news out here in the ’burbs is that some folks are waiting until they come to a red light to use their electronic devices. It is obvious, because heads immediately drop when cars stop rolling.

The bad news is that these conscientious drivers manage to tie up traffic at the red lights. On a drive across Loudoun County, I saw this at every light and had to give a toot-toot-toot to call drivers’ attention back to the road.

Why can’t people pull over and park if they need to check a map, e-mail or make a phone call?

The latest behavior, while safer, is still a bad habit that annoys attentive drivers who do not factor stupidity delays into their travel time line.

Helen Ganster, Leesburg

DG: Maybe the Texas Transportation Institute needs to develop a stupidity index to measure the role of distracted driving in traffic congestion.

A driver can be stopped in traffic but still distracted, which can have consequences for other travelers. And that’s not just the annoyance of waiting behind a green light for a driver who is not moving.

There’s so much going on at intersections. Maybe someone is running a red light just when you’ve got the green signal. Perhaps a pedestrian is sprinting across before traffic starts to move. Drivers who have been consulting a map, an e-mail or a set of online directions rarely snap back to give full attention to their surroundings. Meanwhile, they might be hitting the gas. It’s not a good combination of activities for safety.

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