Adding Highway Lanes Will Reduce Congestion Despite The Skeptics

I-5 traffic before 4:oo pm.

Oregon Department of Transportation TripCheck camera

Will adding lanes to a highway reduce congestion? That argument has been made in my town, and that was an argument many years ago when I lived in Florida and widening was proposed for the road to a beach. The argument usually does not stand up, though. People running a business where traffic matters should understand the issue.

Residents in a fast-growing city may see additional lanes become congested just like the road used to be. But the proper question is not, “Did congestion improve when lanes were added?” Instead we should ask, “How does actual congestion compare to what it would have been without the additional lanes?” If an area has strong population and job growth, then more vehicles traveling is natural. If the lack of congestion relief comes simply because the region grew, then ask what the traffic would have been without the extra lanes.

Let’s imagine a city with no growth, at least not regarding travel from the city center to the suburbs. Suppose there are two alternative roads north, Route 1 and Route 2. Now Route 1 gets another lane. Could congestion on this road be just as bad as before the additional lane was built, even with no growth of population or employment? If Route 1 now has the same congestion as before, the total number of cars traveling on Route 1 is higher. What induced those people to drive on Route 1? Previously they had weighed the alternatives: Route 1 or Route 2. Why would anyone shift from Route 2 to Route 1 if congestion is the same as before? That implies irrational behavior. But even if true, the additional lanes on Route 1 have reduced congestion on Route 2.

More likely, some people start using Route 1 instead of Route 2 expecting less congestion. Expectations adjust to reality over time and people are taking the best possible route given their starting point and destination. A balance is reached where congestion on both roads is lower than before the lane addition.

The idea that extra lanes don’t relieve congestion comes because places that add lanes tend to be growing. That growth will increase congestion if new capacity is not built. But people tend to see simply that extra lanes are now as congested as the old lanes before extra construction, without asking how the extra people would have gotten around with the old number of lanes.

Sometimes, though, extra lanes will spark more travel. In the case of a road to the beach, an extra lane might induce people to take trips they otherwise would not have taken. Imagine a large number of inland residents thinking, “I’ll drive to the beach if I can get there in less than 45 minutes.” The volume of beach-goers stabilizes so that a trip takes exactly 45 minutes.

When an extra lane is added, at first trips take just 20 minutes. Many people learn of this and head to the beach. The volume of traffic stabilizes at a level leading to a trip taking 45 minutes. Congestion is the same as before. The person who lives at the beach sees no benefit, but look at the inland residents who now go to the beach more often. If a two-lane road was increased to four lanes, then twice as many people are able to go to the beach. That’s a social benefit in itself. People clearly want to go to the beach. Now more of them are able to do what they would like to do.

Opponents of automobility argue that low congestion—fast highway speeds—will lead more people to drive rather than take public transit or work from home. That’s likely true, though the magnitude in a modern metropolitan area will probably be fairly small.

To the extent that people do respond to the reduced congestion by traveling more, that reflects those people’s preference for how they live their lives. Whether this is better for the community as a whole or not, businesses that depend on traffic will gain. Business leaders working in a local market must understand the area’s underlying growth. And for some businesses, traffic and congestion heavily influence customer traffic.