Eric Pryne of the Seattle Times today provides a welcome but puzzling update on a project we’ve been watching for some time.

The gist:

The 400 volunteers in the Puget Sound Regional Council’s "Traffic Choices" study have been paying virtual tolls since July. Devices mounted on their dashboards track where they travel and transmit the information to a central computer. Charges are deducted from prepaid "endowment accounts."

Those accounts are just play money. But if there’s anything left in them when the experiment ends in February, participants get to keep it—in real dollars.

That’s the carrot. They can save money by not driving as much, by choosing less-congested highways, or by staying off the road at rush hour.

As we’ve argued for years, paying-as-you-go for driving, rather than in occasional lump sums, ought to be a powerful incentive to economize on miles, trips, and fuel.

Here’s the puzzler:

[Study director Matthew] Kitchen says interim results indicate that, as a group, the study’s 400 participants actually are driving a bit more than they did before the experiment started. But they’re also paying a little less than they would have if the tolls had been in force when researchers first began monitoring their driving.

That could mean the volunteers are avoiding roads with the steepest tolls at times, even if it takes them out of their way, he says. "But we may find something very different when we do a more detailed analysis," he cautions.

What’s going on here? You put a by-the-mile price on driving and people do more of it? That’s unexpected!

Study results won’t be complete for months, of course, and these preliminary trends may be a seasonal fluke, a statistical error, or an effect of some unrelated factor such as a fuller employment.

But it could also be that drivers are much quicker to change roads than to change modes, or to reorganize their weekly travel plans. Stated that way, it’s less surprising. Past experience suggests that it takes some time for people to adjust where they go.

Still, by the time the study is done, I’ll have to rethink some assumptions if drivers haven’t reduced their total miles driven as well as their peak-hour trips on high-priced, congested roads.