Jarrett points us to a just-published paper in World Transit Research that contends that Paris’s new T3 light rail line caused traffic congestion on the adjacent freeway, the Boulevard Périphérique, to increase, thereby causing a net increase in environmental damage and a negative social rate of return. Reading it at its original source requires academic access; here is a mirror on this blog, and thanks to ant6n for sending it. The study does not produce much evidence that an increase in traffic congestion indeed happened. As Angus Grieve-Smith explains in the comments on Human Transit:
It’s important to note that the authors did not measure traffic on the Périph. They just observed that average speeds on the highway declined from 45.9 km/h to 43.5 km/h, and that “many witnesses of the public hearing on the extension of the tramway to Porte de la Chapelle testified their fears to see an analogous shift increasing the congestion on Eastern Périphérique.” In other words, bullshit.
The fact is that a large portion of the traffic on the Périph is going from one side of the city to the other. If some of the drivers on the Maréchaux transfered to the Périph, increasing congestion there, some of the drivers on the Périph would take commuter trains across town instead. Some of the drivers would find it more convenient to take the metro instead of the tramway, or to drive an alternate route that doesn´t involve the Périph, possibly one of the parallel boulevards closer to the center of the city.
The study spends very little time arguing that an increase in traffic happened. It almost takes it for granted. The evidence it provides is that the average speed on the entire Périphérique went down 5%, from 45.9 to 43.5 km/h, whereas the average speed on the southern segment, which parallels the T3 line, went down 10%, from 37.9 to 33.9 km/h.
Instead of arguing that the reduction in speed represents extra traffic coming from the lanes removed to make room for the T3, the study assumes that 100% of the reduction in traffic on the Maréchaux, the boulevard on which the T3 runs, was transferred to the Périphérique. This is unlikely: the phenomenon of reduced demand is attested in the literature – see references here. Traffic shifts to less congested times of day, and sometimes disappears entirely as drivers choose not to take the trip. For one example, when the West Side Highway collapsed, about half its traffic disappeared; this percentage is high, presumably because Manhattan has good transit options, just like Paris.
It’s in fact worse than Angus says. Although the paper provides traffic counts on the Maréchaux, it provides no such counts for the Périphérique, although such counts should be very easy to find. Its computation of the traffic increase on the Périphérique comes entirely from prior assumptions about the traffic that disappeared from the Maréchaux. Another, more minor sleight of hand is the choice of years. For the Maréchaux, the paper argues for comparing present traffic to traffic in 2003, just before the tram’s construction began; for the Périphérique, the numbers provided use 2000 as a baseline.
Most of the paper’s effort is spent not on trying to prove that traffic increased, but on computing the social costs and benefits under questionable assumptions. Doing that is difficult to say the least without knowing more about the nature of traffic on the Périphérique, and the study makes even more questionable assumptions there. To be fair, the biggest smoking guns do not concern the social cost that according to the study is by far the highest, slower traffic speeds; those follow from the assumptions. Instead, they serve to showcase a careless and even biased thought process.
First, the difference in carbon emissions between free-flowing traffic at 38 km/h and 34 km/h is small; what causes fuel consumption to rise in traffic jams is not lower average speed but rather stop-and-go traffic. Thus, even a first-order estimate of extra fuel consumption is impossible given the study’s numbers and assumptions. Fortunately for the study, the carbon cost it uses is so low (€25/ton) and the overall effect posited not large enough that the overall magnitude posited is negligible.
Second, in its computation of economic costs, the study makes the following observation about the project’s cost:
Available information on the monetary costs associated with the project is scarce. One has only the ex ante costs envisioned in the official preliminary Public Inquiry: 341.8M€ for the initial investment and 43.9M€ for the exploitation of the tramway. Experience suggests that ex post costs are likely to be appreciably higher (Flyvbjerg et al. 2002).
For the record, it took me all of three minutes to search on Railway Gazette and Google and find ex post costs amounting to €311.5 million. Worse, the paper says it chooses to use the original cost estimate for lack of other numbers, but then multiplies the original budget by 1.3, the standard factor for public projects in France. As far as I can tell, the reason for multiplying budgets by 1.3 is to cushion against small budget overruns, which could turn slightly beneficial projects into net liabilities; it’s a more honest way of including a contingency budget. In other words, the paper claims that costs probably ran over but its cost estimate for net benefit purposes assumes they didn’t, while in reality they didn’t run over while the paper assumes they did.