The Washington Department of Transportation reports that traffic was down about 37 percent on the 520 floating bridge this morning, the first day of post-holiday tolling.

The decline is roughly in line with estimates on the impact tolling would have on the bridge, which costs drivers up to $3.50 — plus a $1.50 surcharge for those without the stickers — during rush hour. When the tolling began Dec. 29, traffic was down even more at nearly 50 percent, but that was during the holiday week between Christmas and New Years.

The decline was in line with estimates because people respond predictably to economic incentives by changing their behavior. That's the notion behind long term plans to implement congestion pricing in the region as a means of regulating traffic—tolling plans critics typically berate as "social engineering."

It's true. But you know what else is a form of social engineering? Freeways.

Providing toll-free roads on the taxpayers' dime also changes behavior, by incentivizing people to drive their cars more often and further distances. You can argue whether that's a good or a bad thing, but it's social engineering nonetheless, and it's the kind of social engineering that has built suburbs and strip malls, and has generally reshaped our nation over the past 75 years or so.

So while you could say that traffic on the 520 bridge is down 37 percent in response to the new tolls, it might be equally accurate to say that traffic had been inflated 37 percent due to the prior lack of tolling.