Latest travel data show why induced demand remains just a theory

By MARIYA FROST  | 
Apr 1, 2020
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Our usually gridlocked highways are unrecognizable today. The COVID-19 outbreak and quarantine have left them eerily vacant. On the one hand, they are now the roads we’ve all been dreaming about while sitting in stop-and-go traffic on our way to work. On the other hand, these empty roads are a reminder of the people we cannot visit, the places we cannot take our children to, and the jobs that have been changed or lost.

As of March 27, the Washington State Department of Transportation (WSDOT) reports traffic volumes have been cut in half or more across the Puget Sound region.

For the week of March 16, the morning commute on I-405 through Bellevue, compared to February data, saw traffic volumes drop by 35 percent. Traffic volumes declined by 45 percent on I-90, and by 60 percent on State Route 520.

After the Governor issued a shelter-in-place order on March 23, traffic volumes plummeted even further. On I-405, traffic volumes dropped by 65 percent, on I-90 by 65 percent, and on State Route 520 by a remarkable 77 percent. Traffic on I-5 in Everett declined by 56 percent.

As WSDOT analyzed the impact of COVID-19 on traffic congestion, they found that commute times on our much emptier roads are still highly sensitive to difficult driving conditions (like rain) or accidents. A crash involving a truck that closed two left lanes in Seattle, though it was cleared by 7am, resulted in a 30 to 40-minute increase in commute times.

WSDOT also notes that “the difference between a smooth commute and a rough one is at the margins. Any specific drop in traffic demand, especially during the peak periods, can result in noticeable improvement in trip time.”

There is another interesting aspect to all that open space on our roads – it highlights the limitations of “induced demand” theory, which is often cited as a reason we shouldn’t build more road capacity.

Induced demand theory says if you supply more of any product, more people will want it. Transit advocates apply the theory to road construction to suggest that building more lanes will simply cause people to drive more, filling up any new capacity, and making traffic congestion even worse than before. In other words, the availability of pavement will induce more trips. One of the problems of the induced demand theory is that it assumes “continuous congestion everywhere,” which you can see for yourself is not true by looking at current traffic volumes on our roads. 

To be clear, when prices go down (whether that be counted in money or time), the quantity demanded goes up. But that demand is not created – it already exists and is simply waiting for the right price. The theory that this demand is created – that otherwise unnecessary trips emerge – is nonsense. Creating more space on the road reduces the time-cost of travel and allows the supply of public roads to accommodate existing, necessary trips.

The current open road is evidence of this, as it has not resulted in more trips because there are other costs to travel beyond the time it takes to drive. Other factors – like a global pandemic – obviously outweigh the availability of pavement. At 2am, the roads are clear because people would rather be sleeping. On the Friday before Labor Day weekend, on the other hand, people are still willing to hit the road despite knowing the drive might be slow.

This unique circumstance and other factors show that road space is not the dominant force in how people choose trips. And, while reducing travel delay is an important metric, we should also consider vehicle and passenger throughput per hour per lane (multiplied by the number of lanes). This represents the people who can now make the trips they need to because the time cost has gone down, thanks to higher road capacity. This additional throughput is a positive outcome that should be pursued by state DOTs.