Nantes

Nantes opened its data portal in 2011, by its own account the sixth French local authority to do so, and later merged it into a platform shared with the department and the region. On a count taken through the catalogue's own interface in August 2026 it held 589 datasets and every one of them carried an open licence — 544 under the national open licence, 43 under the open database licence, two under later variants — with the licence facet summing exactly to the catalogue total, so nothing sits untagged. Twenty publishers account for nearly all of them, and the transit operator, the regional air-quality association, an advertising contractor, a car-share operator and a cycling association all publish under their own names. The catalogue carries working series rather than only registers: hourly car-park occupancy back to 2010, daily cycle counts, an electromagnetic-field observatory. A complete licence record is not the same as a growing one — a trade profile in 2017 counted 859 datasets on the same portal.

The transit feeds show the same city being careful and getting caught anyway. The operator has published a real-time arrivals interface as open data since 2013, and from late 2024 moved to the French national profile of the international transit standard, announcing that the legacy interface would no longer be maintained from December 2025. The national access point that monitors declared mobility feeds reports those endpoints at full availability and both of the operator's general-format real-time resources at zero — unavailable since 4 July 2026, with forty consecutive automated notifications sent to the producer by 10 August. Standardising on the national format is defensible; the feed international journey planners actually consume is the one that is dark. Both real-time endpoints also sit behind a key, so open here means registered rather than anonymous, a distinction the licence label does not make.

What makes Nantes distinctive is that it publishes its own error bars. The métropole runs inductive counting loops in its cycle infrastructure and republishes the daily series each morning — and every record carries the métropole's own estimate of whether that counter was malfunctioning that day. Across 144,454 loop-days it flags 10,889, or 7.5 per cent, as probably faulty, and warns in the dataset itself that faults also produce under- and over-counts its flag is not designed to catch, so year-on-year growth cannot be read off the file without correction. The electromagnetic-field observatory came out of a citizen debate after the city asked operators to suspend their 5G rollout: around fifty battery-powered sensors on lampposts near schools and crèches, each taking a six-minute broadband reading every couple of hours, with 1,204,385 readings published openly. The mean is 0.91 volts per metre against regulatory limits from 37 upward, and no reading triggers anything — the network answers a political question and says so.