Congestion, Pollution and Freeways

Michael Lewyn's picture
A common argument in favor of building sprawl-generating roads and highways is that if we just pave over enough of the United States, we can actually reduce pollution and greenhouse gas emissions by reducing congestion.  For example, a Reason Foundation press release cited a report by two University of California/Riverside engineering professors, "Real-World CO2 Impact of Traffic Congestion" (available online at ).    But if you read the report carefully, its policy impact is a bit more ambiguous.

Using complicated modeling, the paper purports to list CO2 impacts for thousands of freeway trips in congested Los Angeles.  According to the authors, whenever congestion causes cars to travel less than 45 mph, CO2 emissions increase.  Thus, reducing congestion to prevent bumper-to-bumper traffic will reduce emissions.  But the same paper asserts that "If moderate congestion brings average speeds down from a free-flow speed of about 65 mph to a slower speed of 45 to 50 mph, this moderate congestion can actually lower CO2 emissions." (Id., p. 9).  Moreover, CO2 emissions increase quite rapidly at speeds above 65 mph. (Id., p. 11).

So do these apparent facts support an avalanche of new or widened freeways?  Not necessarily.  On most freeways most of the time, people travel at speeds far above the 45 mph ideal.  The paper points out that even in notoriously congested Los Angeles County, "speeds around 65 to 70 mph dominate." (Id., p. 13).  

It logically follows that even in Los Angeles, it is unclear whether a freeway "improvement" would do more good or more harm.  Assuming that the improvement actually reduced congestion,* some highly polluting bumper-to-bumper traffic would be eliminated (good news) but some 45-50 mph trips might be turned into more-polluting 70 mph trips (not-so-good news).  

It may also follow that where congestion is less frequent (for example, my current residence in Jacksonville, Fla.) an improved highway would be even more likely to increase pollution than in Los Angeles.  Why?  Because in Jacksonville, there are fewer slow trips for a road improvement to eliminate.  Thus, a freeway widening would be more likely than in Los Angeles to lead to more "bad news" trips (in which speeds increased from 45-50 mph to 70 or more mph) than "good news" trips (in which speeds increased to 45-50 mph from a lower speed).

*Given the amount of controversy over "induced traffic" (the idea that wider roads tempt people to drive more, thus eliminating congestion gains over the long run), I am not sure this is the case.
Michael Lewyn is an assistant professor at Touro Law Center in Long Island.



A More Reason-able Conclusion

If their goal were really to reduce GHG emissions, they would support lowering the speed limit to 45 or 50 mph on existing freeways at all times, rather than building new freeways.

Charles Siegel

It all depends

First, why focus on just CO2 emissions? CO, though not much of a problem anymore, is far less of an issue at higher speeds. Nox is complicated (nonlinear correlation with speed), most pollution on balance, is better with less stop-n-go and moderate speeds.

As far as building more roads to reduce congestion and pollution, it's a catch 22. If you assume that driving is and will be the top preference of travel mode, their argument has some merit, because this just makes driving less painful and might actually reduce pollution (I don't buy the induced travel argument because roads don't make people have kids and the mode split is so car heavy already, it wouldn't have much impact).

On the other hand, if your ultimate strategy is to increase density and precipitate mode switch out the car, that could be better, but you need multiple things to work together. The problem now is of course, politicians will simultaneously subsidize downtown development, call for densification of land use, get money for new roads, extend transit lines, and allow NIMBYs to influence them to artificially keep low densities. It's the ultimate yes/no answer. This is why I would love the potential effects of a market based land use transportation system. The political influences of the constant yes/no response provide no coherent outcome.

Pollution and Stop-And-Go

"most pollution on balance, is better with less stop-n-go and moderate speeds."

CP, are you aware of Newman and Kenworthy's international comparison showing that local air-pollution emissions are lower in cities that have more congestion?

That conclusion is counter-intuitive, because there are more emissions per mile driven with stop-and-go traffic. But N and K show that this is balanced by the fact that 1) people drive less if traffic is slower and 2) densities are higher in the cities with more stop-and-go traffic, which also leads people to drive less.

That is just urban air pollution within the local region. Given the looming problems of global warming and peak oil, I think it is more important to focus on co2 emissions/energy consumption at this point in history.

"I don't buy the induced travel argument because roads don't make people have kids and the mode split is so car heavy already, it wouldn't have much impact"

Regardless of mode split, speed affects the distance people travel. If average speed is 70 miles per hour, you are more likely to go shopping in a distant mall or to move to a remote suburb where you have a long commute than if average speed is 20 miles per hour.

Charles Siegel

Fair points

Certainly, people may drive some additional miles with less traffic and higher speeds, but I would argue that much like we've had already, you would have increased job and retail suburbanization with the added highways approach.

Either way, I think it is clear (as we've discussed before) that public subsidy of any motorized/mechanized transportation just leads to excess use, excess energy consumption, a larger urbanized area, and a more damaging ecological footprint overall.

The Reason argument in this case may in fact reduce congestion, but I don't think it would have any overall environmental benefits (would probably make it worse). Their proposal to privatize highways or lease them does have merit in my opinion because it would be the first step in charging drivers for their use of facilities. Then, the additional external cost imposed by auto pollution would rest upon the public sector. Alternatively, they could jack up lease rates high enough so private companies would be forced to pass along a higher than direct cost fee to drivers assuming it would still be financially feasible at those rates.

Michael Lewyn's picture

A technical note

I focused on CO2 because the study at issue focuses on CO2.

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