Air quality, congestion, and pricing

One of my former students posted this story from the Wall Street Journal to Facebook yesterday. ARGH. Does the WSJ do no research in its reporting any more? So a time delay will shift drivers to transit but a charge for driving wouldn’t? Are WSJ reporters not required to understand basic econ anymore?

Congestion can be either good or bad for air quality depending on the geographic distribution of alternatives, the fleet, and the context into which the emissions enter. Congestion charging can in theory price trips onto the road under hypercongestion (the famous backward-bending demand curve), and thus may allow for greater VMT. But this is not true even in theory if the charging scheme is priced for VMT reduction or rationing rather than optimizing flow–those are different things, and the New York pricing plan was zone-based, so it most likely would have eliminated trips, not increased them! Higher throughput might affect emissions via flow optimization, but it is not apparent from either monitors or lab experiments that idling among a smaller number of cars/trips is objectively better for air quality than fast throughput from a larger number of cars/trips (and there may be a substitution effect in the traveling fleet that most people think is an air quality benefit; or there may be a freight effect which would not produce benefits [1]; we don’t know). Different types of emissions also vary with engine cycle, so it may be wash in total but a trade in emissions type.

Studies prior to the implementation of London’s cordon toll predicted fairly large reductions in emissions [2]. After the charge’s implementation, monitors throughout the metro area demonstrated statistically significant (though small in absolute terms) and progressively distributed improvements in pollutant concentrations [3,4]. Now, it’s not likely that we can attribute all of that change to the cordon toll, as accompanying the toll were rapid increases in bus supply (so much so that train trips also went down concurrently following the implementation of the cordon toll; I suspect that congested trains were traded for less-congested buses for short trips, particularly after the transit bombings in July of 2005).

Traffic calming, transit-oriented, density-based efforts in Paris—without the concomitant tolling efforts that London put down—have been found to increase emissions substantially, so New York may not be doing itself any favors with unpriced congestion [5]. Concentrations in some congested areas, like San Francisco, have not been improving as quickly as in other places. It may be that these simply reflect that when air quality is generally good in high-growth areas, reductions occur marginally more slowly than in places where concentrations are comparatively high. (IOW, it’s hard to improve on good.) However, it can also be that worsening congestion has caused higher emissions and that is being reflected in some regional monitors.

Some of the smartest transportation people in the world are at New York universities, but why would we actually CALL THEM to get some ideas for this WSJ peice? For most of these people, it wouldn’t even a long-distance phone call for the WSJ reporter. See? Look at this list! This is a dream team of people to ask rather than just talking out of your backside. BAD WSJ.

But then, as my friend Chris Redfearn once noted, discussions like this require people to keep more than one idea in their heads at once. If we are throwing around the “dubious” term, congestion on transit–and this exists virtually everywhere in the world that isn’t in the US–is most definitely NOT good for the environment: see Mexico City, see virtually every Asian city over 10 million people, and the many scooter engines on the road rather than transit trips.

[1] Marr, L. C. and R. A. Harley. 2002. Modeling the effect of weekday-weekend differences in motor vehicle
emissions on photochemical air pollution in central california. Environmental Science and Technology, 36:4099–4106.

[2] Beevers, SD, Carslaw DC. The impact of congestion charging on vehicle emissions in London. Atmospheric Environment. 2005 Jan ;39(1):1-5.

[3] Atkinson R, Barratt B, Armstrong B, Anderson H, Beevers S, Mudway I, et al. The impact of the Congestion Charging Scheme on ambient air pollution concentrations in London [Internet]. Atmospheric Environment. In Press, Accepted.

[4] C, Beevers S, Armstrong B, Kelly F, Wilkinson P. Air pollution and mortality benefits of the London Congestion Charge: spatial and socioeconomic inequalities. Occup Environ Med. 2008 Sep 1;65(9):620-627.

[5] Bouf D, Hensher DA. The dark side of making transit irresistible: The example of France. Transport Policy. 2007 Nov;14(6):523-532.