We just discovered I-5 solution

In the case of the vanishing cars, the plot thickens.

The state has reopened our busiest freeway, Interstate 5, after a two-week partial shutdown. The dawn-to-dusk traffic apocalypse never arrived. But what we still don't really know is: Why? How? Where in the world did all the cars go?

The theories are as plentiful as empty seats will be this week on the Sounder trains. It's been said that people left on vacation. Or telecommuted. Or took mass transit in droves.

It turns out none of these is true. At least not in big enough numbers to explain the smooth driving these past two weeks.

Take the transit myth. If you add all the trains, buses, van pools and water taxis, about 2,500 people per day hopped a ride who don't usually.

That's fine, but I-5 carries 130,000 cars per day just in the northbound lanes. Yet traffic was down there as much as 50,000 cars a day. Transit can account for maybe 5 percent of these missing cars.

So I asked two smart traffic people: Where did the cars go?

Their answers suggest a way to solve our traffic mess. Without spending much money. And without building new freeways.

I asked that Redmond math whiz, Oliver Downs, the guy who predicted there'd be no gridlock. His firm, Inrix, gets official traffic counts as well as instant data from thousands of cars out driving local roads.

He compared traffic the week before I-5 was shut down with the project's first week. He found that, across all highways in and around Seattle, traffic dropped only 4 percent.

"It's fascinating," he says. "The cars didn't vanish at all. They diffused."

What does this mean? Dave McCormick, of the state Department of Transportation, says people drove differently.

"The traffic mostly shifted and spread," he says — shifted slightly to other routes, and spread substantially to other times of day.

So it wasn't that folks left town or flocked to transit or stopped running errands. They changed how and when they drove, but they kept on driving.

So here's my idea: Let's replicate what just happened. We already restrict cars from using Third Avenue downtown at rush hours. Now let's do that on the major freeways.

We could set up a license-plate cordon around downtown Seattle. Drivers with, say, even plate numbers could drive in at rush hour some days, while those with odd numbers get the privilege the other days.

It could be enforced with license-plate cameras and a small fine. If it's not your day, you'd have to take transit, car-pool with someone who has privileges that day or, as in these past two weeks, alter your route or shift to a non-rush-hour time. Commercial plates could be exempt.

There's no toll, so it's equitable. It's practically cost-free. It would instantly ease congestion without new freeways. It would likely allow us to tear down the Alaskan Way Viaduct. And it would free up money for better transit and for repairing the dilapidated roads we've already got.

It's your choice. The current plan is to spend the most money ever asked of local taxpayers and still not solve the problem. Or, as you just did, you could try something else.

Danny Westneat's column appears Wednesday and Sunday. Reach him at 206-464-2086 or dwestneat@seattletimes.com.