We can calculate how much added freeway lanes will induce additional car travel

The takeaway:  the I-5 freeway widening project in Portland lead to 10 to 17 million more miles of vehicle travel annually, which will in turn produce thousands of tons of additional greenhouse gas emissions.

A key part of the selling point for the proposed $500 million Rose Quarter Freeway widening project is the improbable claim that widening the freeway will reduce traffic congestion and not stimulate additional vehicle travel. That claim flies in the face of decades of experience and widely published research showing that, with great predictability, more freeway capacity generates proportionately more traffic, traffic congestion and pollution. The phenomenon is now so well established in the literature that it is called the “Fundamental Law of Road Congestion.”

It’s also so well-established that researchers at the University of California Davis have built an on-line calculator that allows you to compute how many more vehicle miles of travel a wider freeway will produce.  Their calculator is based on a careful review of the research on induced demand, and generates an estimate of the number of additional vehicle miles of travel that will be produced by each additional lane-mile of freeway in a city. Here’s the calculator showing how much adding 1.6 lane miles of freeway would increase travel in the San Diego metropolitan area (about 11 million miles per year).


Because it’s based in California, the model is calibrated for California metro areas and not Portland. To get a rough idea of what kind of impact we might expect from a freeway widening project in Portland based on this model, we looked at the model’s computations of the impact of freeway widening projects in the three California metropolitan areas most similar in population size to Portland–Sacramento, San Diego and San Jose).  For each city, we used the calculator to generate estimates of the additional vehicle miles of travel associated with adding 1.6 lane miles of interstate freeway.  Here are the results:  in these California cities, adding 1.6 lane miles of freeway would be expected to generate between 8 million and 12 million additional miles of vehicle travel.

Of course, Portland isn’t exactly like any of these three cities.  So using the data from the model, we developed our own estimate of the likely effects of induced demand from freeway expansion in Portland.  The section of I-5 that would be widened as part of this project carries about 120,000 vehicles per day, and has four lanes in each direction.  According to the Environmental Assessment Traffic Technical Report, page 9), the segment to be widened is about 4,300 or 0.8 miles long; adding two lanes therefore adds about 1.6 lane miles of freeway. The Induced Travel Calculator shows that there is a unit-elasticity of vehicle miles traveled to added capacity:  Therefore a 1 percent increase in lane-miles generates a 1 percent increase in freeway travel.

We applied this unit elasticity factor to the expansion of the freeway in the Rose Quarter area.  This .8 mile stretch of freeway currently has about 96,000  vehicle miles of travel daily ((120,000 *0.8), or about 35 million vehicle miles of travel per year.  This project increases the number of lane miles of freeway by 50 percent, from 3.2 lane miles (4 * 0.8) to 4.8 lane miles (6 * 0.8).  The unit elasticity of lane miles to vehicle miles traveled means that a 50 percent increase in lane-miles should produce a 50 percent increase in vehicle miles traveled or about 17.5 million additional vehicle miles of travel.

This estimate is higher than the estimate for the three California cities presented above.  It’s important to keep in mind that the results from the calculator are based on the average traffic volume on all freeways in a metro area; because this area is much more central and more heavily used, it has a higher base of traffic than the typical freeway in a metro area.  Taken together, the evidence from the UC Davis calculator and our application of its methods to the Rose Quarter project, we estimate that widening the I-5 freeway by adding one lane for about 0.8 of a mile would be expected to add between 10 million and 17 million additional vehicle miles of travel in the Portland metro area.

We can go one step further, and estimate approximately how much additional greenhouse gas emissions will results from this added driving. Transportation planning firm Fehr and Peers has yet another calculator for converting additional vehicle miles of travel into greenhouse gas emissions, with estimates calibrated to Western US metropolitan areas. Their calculator suggests that each additional thousand miles of driving is associated with about .466 tons of greenhouse gases; that means the Rose Quarter freeway widening project will produce between 4.7 and 7.9 thousand tons of additional greenhouse gas emissions per year.

It doesn’t matter what you call the added lanes

And we don’t buy for a minute that it matters in any way that ODOT wants to call the additional lanes its building “auxiliary lanes”.  If the point is that the right hand lane on I-5 at the Rose Quarter is handling merging traffic, that is true whether the facility is 2 lanes in each direction or three.  If we apply ODOT’s logic and nomenclature to the current setup, the freeway now consists of one through lane and one auxiliary lane–and the proposed project would increase that to two through-lanes and one auxiliary lane. Using sophistry and shifting definitions doesn’t change the fact that this project adds lane miles of freeway. And more lane miles of freeway, as these calculators show, produces millions more miles of driving and thousands of tons more greenhouse gas emissions every year.