Skip to Content
Streetsblog USA home
Streetsblog USA home
Log In
Around the Block

The Science Is Clear: More Highways Equals More Traffic. Why Are DOTs Still Ignoring It?

10:32 AM EDT on June 21, 2017

Numerous studies have documented the phenomenon known as induced demand in transportation: Basically, if you build highway lanes, more drivers will come. And yet, transportation agencies rarely account for this effect when planning road projects.

In a recent paper published by the Transportation Research Record, author Ronald Milam and his research team reviewed the various studies documenting the induced demand effect. They found that for every 1 percent increase in highway capacity, traffic increases 0.29 to 1.1 percent in the long term (about five years out), and up to 0.68 percent in the short term (one or two years). One recent study found a one-to-one relationship between new highway lane capacity and traffic increases.

However, highway planners are failing to incorporate this effect into their models. Milam told Streetsblog that "it is rare to find an induced travel analysis in most transportation infrastructure design or environmental impact analysis." That means transportation agencies are green-lighting money for highway expansions that are destined to become congested again only a short time later.

While some transportation agencies may lack the sophistication to model induced demand accurately, they should not disregard the phenomenon, Milam and his team write.

Highway planners should assume traffic increases will result from highway expansion projects in line with what previous research has demonstrated, the authors conclude.

More recommended reading today: nextSTL publishes a poem about the epic struggle to convince the Missouri Department of Transportation to design a walkable Gravois Street. ATL Urbanist reports MARTA will take over operations of the struggling Atlanta Streetcar. And Streets.mn looks at the relative dearth of retail stores in urban locations in the Twin Cities.

Stay in touch

Sign up for our free newsletter

More from Streetsblog USA

Why We Care About Some Transportation Tragedies More Than Others

Why do we respond to major transportation disasters with so much urgency — and why don't we count our collective car crash epidemic among them?

March 28, 2024

The Toll of History: MTA Board Approves $15 Congestion Pricing Fee

New York City's first-in-the-nation congestion pricing tolls are one historic step closer to reality after Wednesday's 11-1 MTA board vote. Next step: all those pesky lawsuits.

March 28, 2024

Take Thursday’s Headlines Home, Country Roads

Heat Map reports on why rural Americans are resisting electric vehicles, and why it might not matter much for the climate.

March 28, 2024

Guest Commentary: Traffic Engineers Must Put Safety Over Driver Throughput

No other field would tolerate this level of death and destruction. The tragedy of West Portal is more evidence that the traffic engineering profession is fundamentally broken.

March 27, 2024

Wednesday’s Headlines Missed Connection

The Biden administration is spending billions to reconnect neighborhoods torn apart by urban freeways. But the projects seem to simply paper over the problem, Governing reports.

March 27, 2024
See all posts