While trying to hunt down some numbers on the costs of the three new U5 stations, I found media discourse in Berlin about the U-Bahn expansion plan that was, in effect, greenwashing austerity. This is related to the general hostility of German urbanists and much of the Green Party (including the Berlin branch) to infrastructure at any scale larger than that of a bike lane. But the specific mechanism they use – trying to estimate the carbon budget – is a generally interesting case of knowing the costs more certainly than the benefits, which leads to austerity. The underlying issue is that mode shift is hard to estimate accurately at the level of the single piece of infrastructure, and therefore benefit-cost analyses that downplay ridership as a benefit and only look at mode shift lead to underbuilding of public transport infrastructure.
The current program in Berlin
In the last generation, Berlin has barely expanded its rapid transit network. The priority in the 1990s was to restore sections that had been cut by the Berlin Wall, such as the Ringbahn, which was finally restored with circular service in 2006. U-Bahn expansion, not including restoration of pre-Wall services, included two extensions of U8, one north to Wittenau that had begun in the 1980s and a one-stop southward extension of U8 to Hermannstrasse, which project had begun in the 1920s but been halted during the Depression. Since then, the only fully new extension have been a one-stop extension of U2 to Pankow, and the six-stop extension of U5 west from Alexanderplatz to Hauptbahnhof.
However, plans for much more expansive construction continue. Berlin was one of the world’s largest and richest cities before the war, and had big plans for further growth, which were not realized due to the war and division; in that sense, I believe it is globally only second to New York in the size of its historic unrealized expansion program. The city will never regain its relative wealth or size, not in a world of multiple hypercities, but it is growing, and as a result, it’s dusting off some of these plans.
Most of the lines depicted in red on the map are not at all on the city’s list of projects to be built by the 2030s. Unfortunately, the most important line measured by projected cost per rider, the two-stop extension of U8 north (due east) to Märkisches Viertel, is constantly deprioritized. The likeliest lines to be built per current politicking are the extensions of U7 in both directions, southeast ti the airport (beyond the edge of the map) and west from Spandau to Staaken, and the one-stop extension of U3 southwest to Mexikoplatz to connect with the S-Bahn. An extension to the former grounds of Tegel is also considered, most likely a U6 branch depicted as a lower-priority dashed yellow line on the map rather than the U5 extension the map depicts in red.
The carbon critique
Two days after the U5 extension opened two years ago, a report dropped that accused the proposed program of climate catastrophe. The argument: the embedded concrete emissions of subway construction are high, and the payback time on those from mode shift is more than 100 years.
The numbers in the study are, as follows: each kilometer of construction emits 98,800 tons of CO2, which is 0.5% of city emissions (that is, 5.38 t/person, cf. the German average of about 9.15 in 2021). It’s expected that through mode shift, each subway kilometer saves 714 t-CO2 in annual emissions through mode shift, which is assumed to be 20% of ridership, for a payback time of 139 years.
And this argument is, frankly, garbage. The scale of the difference in emissions between cities with and without extensive subway systems is too large for this to be possibly true. The U-Bahn is 155 km long; if the 714 t/km number holds, then in a no U-Bahn counterfactual, Berlin’s annual greenhouse gas emissions grow by 0.56%, which is just ridiculous. We know what cities with no or minimal rapid transit systems look like, and they’re not 0.56% worse than comparanda with extensive rapid transit – compare any American city to New York, for one. Or look again at the comparison of Berlin to the German average: Berlin has 327 cars per 1,000 people, whereas Germany-wide it’s 580 and that’s with extensive rapid transit systems in most major cities bringing down the average from the subway-free counterfactual of the US or even Poland.
The actual long-term effect of additional public transport ridership on mode shift and demotorization has to be much more than 20%, then. It may well be more than 100%: the population density that the transit city supports also increases the walking commute modal split as some people move near work, and even drivers drive shorter distances due to the higher density. This, again, is not hard to see at the level of sanity checks: Europeans drive considerably less than Americans not just per capita but also per car, and in the United States, people in New York State drive somewhat shorter distance per car than Americans elsewhere (I can’t find city data).
The measurement problem
It’s easy to measure the embedded concrete of infrastructure construction: there are standardized itemized numbers for each element and those can be added up. It’s much harder to measure the carbon savings from the existence of a better urban rail system. Ridership can be estimated fairly accurately, but long-term mode shift can’t. This is where rules of thumb like 20% can look truthy, even if they fail any sanity check.
But it’s not correct to take any difficult to estimate number and set it to zero. In fact, there are visible mode shift effects from a large mass transit system. The difficulty is with attributing specific shifts to specific capital investments. Much of the effect of mode shift comes from the ability of an urban rail system to contribute to the rise of a strong city center, which can be high-rise (as in New York), mid-rise (as in Munich or Paris), or a mix (as in Berlin). Once the city center anchored by the system exists, jobs are less likely to suburbanize to auto-oriented office parks, and people are likelier to work in city center and take the train. Social events will likewise tend to pick central locations to be convenient for everyone, and denser neighborhoods make it easier to walk or bike to such events, and this way, car-free travel is possible even for non-work trips.
This, again, can be readily verified by looking at car ownership rates, modal splits (for example, here is Berlin’s), transit-oriented development, and so on, but it’s difficult to causally attribute it to a specific piece of infrastructure. Nonetheless, ignoring this effect is irresponsible: it means the carbon benefit-cost analysis, and perhaps the economic case as well, knows the cost of everything and the value of little, which makes investment look worse than it is.
I suspect that this is what’s behind the low willingness to invest in urban rail here. The benefit-cost analyses can leave too much value on the table, contributing to public transport austerity. When writing the Sweden report, I was stricken by how the benefit-cost analyses for both Citybanan and Nya Tunnelbanan were negative, when the ridership projections were good relative to costs. Actual ridership growth on the Stockholm commuter trains from before the opening of Citybanan to 2019 was enough to bring cot per new daily trip down to about $29,000 in 2021 PPP dollars, and Nya Tunnelbanan’s daily ridership projection of 170,000 means around $23,000/rider. The original construction of the T-bana cost $2,700/rider in 2021 dollars, in a Sweden that was only about 40% as rich as it is today, and has a retrospective benefit-cost ratio of between 6 and 8.5, depending on whether broader agglomeration benefit are included – and these benefits are economic (for example, time savings, or economic productivity from agglomeration) scale linearly with income.
At least Sweden did agree to build both lines, recognizing the benefit-cost analysis missed some benefits. Berlin instead remains in austerity mode. The lines under discussion right now are projected between 13,160€ and 27,200€ per weekday trip (and Märkisches Viertel is, again, the cheapest). The higher end, represented by the U6 branch to Tegel, is close to the frontier of what a country as rich as Germany should build; M18 in Paris is projected to be more than this, but area public transport advocates dislike it and treat it as a giveaway to rich suburbs. And yet, the U6 branch looks unlikely to be built right now. When the cost per rider of what is left is this low, what this means is that the city needs to build more infrastructure, or else it’s leaving value on the table.