There’s controversy in Germany over building U- and S-Bahn extensions, in which environmentalists argue against them on the grounds that people can just take trams and the environmental benefits of urban rail are not high. For example, the Ariadne project prefers push factors (green regulations and taxes) to pull factors (building better alternatives), BUND opposes U- and S-Bahn expansion, and a report endorsed by Green politicians argued based on shoddy analysis leading to retraction that the embodied carbon emissions of tunneling exceeded any savings, which it estimated at only 714 t-CO2 per underground km built. Against this, it’s important to sanity-check car and public transport ridership to arrive at more solid figures.
To start with, virtually everyone travels by car or by public transport. There’s a notable exception for cycling, but cycling is typically done at short ranges, and the metro expansions under discussion here (all outside the Ring) are beyond that range. In Berlin, the modal split for cycling peaks in the 1-3 km range and is small past 10 km. Beyond the scale of a neighborhood or maybe a college town, cars and mass transit are substitutes for each other.
Nor does public transport expansion lead to hypermobility, in which overall trips grow longer as people commute from farther away and car use doesn’t decline or only weakly declines. If anything, the ratio of substitution for passenger-km rather than trips is that a p-km by metro substitutes for more than one p-km by cars, because metro-oriented cities can be denser and allow for shorter commute trips. Berliners average 3.3 4.6-km trips per day, or 15 km/day; Germany-wide, it’s 35.5 km/day (see table 11 of MiD). If anything, the presence of a large city core also shortens the average car trip by reducing exurb-to-exurb driving at low density.
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Alon Levy
