A new Potomac River bridge is a bad idea, even for drivers, a study shows

People have been pushing the idea of a new highway connecting Gaithersburg to Dulles for decades. Groups like the Coalition for Smarter Growth have been saying it's a terrible idea for just as long. A recent analysis from the regional Transportation Planning Board says it's... indeed quite a terrible idea. 

It's not just bad planning; it actually won't help drivers, either. It would just reduce the total hours of vehicle delay — basically, traffic congestion — by a mere three percent. The average trip time would not go down at all.

The highway, running over a new bridge, would actually increase (by one percent) the per capita vehicle miles traveled, or how much driving each person does, even though some people would get a more direct road for their commutes.

Image by TPB.

That's because while some people would get a shorter commute, far more people would move to new ex-urban housing requiring a longer commute.

Not only would those folks fill up the new road, but the change in trip patterns would also make surrounding roads more congested than they are now, as people drive to access the new highway and increase sprawl along it.

The "U-shaped commute" via the Beltway from Gaithersburg to Dulles, which road supporters often cite, would get easier. However, very few people actually make that drive today. Meanwhile, vastly more long new drives would be induced. 

The end result would be that the region would reach a new equilibrium with even more driving and traffic than now.

This is called induced demand, or the Fundamental Law of Road Congestion.

Put simply, the Fundamental Law of Road Congestion says, "roads cause traffic." Or, less simply as described by some researchers in Toronto and London (so they used metric):

The number of vehicle-kilometers traveled (VKT) increases in direct proportion to the available lane-kilometers of roadways. The additional VKT traveled come from increased driving by current residents and businesses, and migration.

Building new roads and widening existing ones only results in additional traffic that continues to rise until congestion returns to the previous level. Such attempts to “cure” congestion are thus both expensive and ineffective.

Increasing the lane kilometers for one type of road does not significantly reduce congestion on others — for example, widening highways does little to reduce local congestion.

Metropolitan areas appear to construct new lane-kilometers of roadway “with little or no regard for the prevailing level of traffic.”

Because roadways have “natural” levels of congestion to which they always return, mass transit projects will not reduce traffic.

Image by Dome Poon licensed under Creative Commons.

To many people, this is counter-intuitive. The typical person thinks of traffic like a liquid. We even talk about it "flowing." There's some water, and it's going to find a path downhill. If you provide a nice channel, it'll flow there. If you don't, it'll squirt around the sides and get into all the neighborhood streets where we don't want it. So we should build enough roads for the projected needs!

In truth, traffic is more like a gas. It expands to fill the available space. Sometimes the air might flow into neighborhood streets, but lowering the pressure on the main road at best relieves this temporarily until the gas spreads out again.

Most people would love to simultaneously live on a big property, somewhere cheap, and also have a very short trip to work and other destinations. If someone built a big highway right to a new housing complex of large lots which don't cost a lot, the majority of people would move there. However, that's not the tradeoff.

Instead, people could have more space in farther suburbs, but at the cost of more travel time, so some people don't live there. If you add road space, that just shifts the equilibrium, people move there, and the traffic gets worse until it's bad enough to stop people again.

Image by Unknown, 1885 licensed under Creative Commons.

The city is an hour wide, and always has been

In fact, all forms of transportation have the same effect. Transportation makes cities larger but doesn't actually make your commute shorter. This principal is called Marchetti's Constant. Italian physicist Cesare Marchetti noticed that from ancient Rome to today, cities (then) or metro areas (now) are about the size you can traverse in an hour, or 30 minutes to the center. If it's on foot, that's one size; on a horse, another; streetcar, another; cars, another.

As transportation technology developed, we moved farther from the center but not really farther temporally. That's because most people don't want to spend more than an hour of their day going to and from work. So, they don't. But if they can live farther out and still be 30 minutes from work, they will.

Does this mean there is no benefit in building transportation? No; it's just that the benefit isn't in saving you time. The benefit is in economic development. An ancient city 30 minutes wide on foot can only have so many people and so much economic activity. A modern one has more.

The reason to build transportation is for the economic growth it generates, not the time savings. That's why it's frustrating when some people criticize transportation projects as really being for economic development. Every transportation project is for economic development.

Image by Canaan Merchant.

What's the difference between kinds of transportation?

The reason roads, transit, and other modes differ is in the kind of economic development they generate, and other factors like the effect on the environment, which are not the same.

Transit spurs growth that looks like walkable urban places right around transit nodes. Roads generate suburban sprawl-type development. One is far more space-efficient and environmentally-friendly, but people's aesthetic views on these differ, which leads them to make various arguments for why we should build something.

I think of it like irrigating some land where nothing grows. If you build a transportation irrigation pipe out to a fallow field at the edge of your region, houses will grow there. If the pipe is a road, then a car-oriented form of land use sprouts up. If the pipe is a train, then a transit-oriented form sprouts up. If you instead irrigate an area near a region's core with better buses and bike lanes, things like apartments and row houses sprout up.

These decisions have effects on the exiting residents, too. Water an exurban greenfield cul-de-sac housing subdivision and it worsens the traffic for everyone living closer in or nearby. This is a way transit is different from automobility; with transit, the more people ride, the better for existing users (to a point which we usually don't reach). With car-oriented growth, it's the opposite; the more people drive, the worse for you.

We tend to think about transportation backward. We think about it as transportation, which leads to questions like, "Where do people need to go?" and "Where is there congestion?" These questions just lead to answers that circularly re-create the same problems.

Instead, we should ask, "Where do we want future people to live, work, and shop?" and "How do we want them to get there?" Then we should make transportation investments with those in mind.

Not the bridge

The outer Potomac bridge and highway would water the wrong things. That's why even by objective metrics of transportation, it won't help.

On Wednesday, a committee of the regional Transportation Planning Board recommended excluding the concept from its long-term future transportation recommendations. The committee did recommend five of ten projects: balancing out land use, Transportation Demand Management, transitways with dedicated lanes (including Bus Rapid Transit), core Metro, and HOT lanes. The bridge, which was one of the worst performers in the study, was clearly out.

The region has studied this highway and bridge concept repeatedly over the years. It's come up short each time. This new study offers one of the clearest answers yet. Let's start focusing on transportation improvements, and by extension economic development consequences, which move the region in the right direction.

Comment on this article