Overall, if you compare these two pictures, it’s pretty clear that November 12’s traffic situation in downtown Seattle was much better than a typical day. Sure, Interstate 5, the freeway to the East of downtown Seattle is congested (as it is most late afternoon weekdays). But downtown Seattle streets, particularly on the west side of downtown are “green,” or free flowing. Overall, there’s a lot more “green” on November 12’s traffic charts than on a typical day. In other words: no gridlock or Carmaggedon here.

Tolling the new SR 99 Tunnel didn’t make traffic worse. If anything, it made traffic better. The tunnel itself was flowing smoothly—it was green rather than red, meaning those paying the toll were getting value for their money. Not only that, but it doesn’t appear that traffic on downtown streets in Seattle was any worse than on an ordinary Tuesday. It’s likely that by reducing travel volumes on the tunnel, tolling reduced the number of cars driving onto downtown Seattle Streets. Small reductions in travel demand that keep roads from crossing a tipping point and becoming congested make traffic move more smoothly.

If this gives you a bit of deja vu, dear reader, it should. Back in January, just before the tunnel opened, the city had to commence demolition of the old viaduct, in order to connect on ramps to the new tunnel. As a result, the city suddenly lost its old waterfront freeway, and didn’t have access to the new tunnel. State highway officials warned that the city was in for weeks of gridlock. But when they closed the viaduct, not only did nothing happen, but as I related at City Observatory, traffic on most of downtown Seattle got better. Rather than simply diverting to other city streets, traffic levels went down; as the Seattle Times reported “traffic just disappeared.”

What road closures teach us about travel demand

So what’s going on here? Arguably, our mental model of traffic is just wrong. We tend to think of traffic volumes, and trip-making generally, as inexorable forces of nature. The diurnal flow of traffic on urban roadways seems just as regular and predictable as the tides.

What this misses is that there’s a deep behavioral basis to travel. Human beings will shift their behavior in response to changing circumstances. If road capacity is impaired or priced, many people can choose not to travel, change when they travel, change where they travel, or even change their mode of travel. The fact that Carmageddon almost never comes is powerful evidence of induced demand: people travel on roadways because the capacity is available for their trips, when when the capacity goes away or its price goes up, trip-making changes to reduce traffic.

If we visualize travel demand as a fixed, irreducible quantity, it’s easy to imagine that there will be Carmaggedon whenever a major link of the transportation system goes away. But in the face of a changed transportation system, people change their behavior. And while we tend to believe that most people have no choice for when and where they travel, the truth is many people do, and that they respond quickly to changes in the transportation system and to road pricing. It’s a corollary of induced demand: when we build new capacity in urban roadways, traffic grows quickly to fill it, resulting in more travel and continuing traffic jams. What we have here is “reduced demand”—when we cut the supply of urban road space, traffic volumes fall.

If Seattle drivers quickly change their behavior in response to a dollar or two of tolls, that’s a powerful indication that more modest steps to price roads don’t really mean the end of the world. If we recognize that, in the absence of pricing, traffic will tend to adjust to available capacity, we then end up taking a different view of how to balance transportation against other objectives. For example, this ought to be a signal that road diets, which have been shown to greatly improve safety and encourage walking and cycling, don’t have anything approaching the kinds of adverse effects on travel that highway engineers usually predict.

There’s one other fiscal codicil to this tale. The tolls that drivers pay to use the new $3 billion tunnel cover barely 10 percent of the costs of construction. The fact that tolls reduce traffic by a third (or more) show that a good fraction of tunnel users are don’t value the tunnel at even a tenth of the cost of providing its right-of-way; and that they’ll use it only if its free. That’s a pretty solid indication that this “investment” has negligible value. In addition, the fact that implementing pricing has caused traffic conditions in much of downtown Seattle to improve is an indication that simply charging a modest price for roadways actually produces net benefits for road users—it encourages those with choices to travel at other times or avoid congested areas, producing better service for everyone. And that may be the real lesson here: the tunnel didn’t make traffic in downtown Seattle better, the toll did.

Carmaggedon never comes

Of course, we’ll wait for detailed data on traffic conditions that will be collected over the next few weeks before making a definitive judgment. But this phenomenon of reduced demand is so common and well-documented that it is simply unremarkable. Whether it was Los Angeles closing a major section of freeway to replace overpasses, or Atlanta’s I-85 freeway collapse, or the I-35 bridge failure in Minneapolis, or the demolition of San Francisco’s Embarcadero Freeway, we’ve seen that time and again when freeway capacity is abruptly reduced, traffic levels fall as well.

There’s a lesson here, if we’re willing to learn it: if you want to reduce traffic congestion, reduce traffic levels. Whether you do it by restricting capacity, or (more sensibly) by imposing tolls that ask motorist to pay for even a fraction of the cost of the roads they’re using, you get a much more efficient system.