Array of Things
Chicago called it a fitness tracker for the city and announced 500 sensor nodes by the end of 2018. About 130 ran, and they were retired in September 2021.
Sensors & IoT is the single most common tag in this entire dataset — more projects carry it than any other topic, "AI" included. That's not surprising once you notice that almost every other tag (traffic, air quality, flood, waste) describes a problem, while this one describes the instrument most of those problems get solved with.
Santander's SmartSantander, an EU-funded testbed running 2010-2013, federated more than 20,000 sensors across four European cities, with 12,000+ deployed under Santander's own streets and lampposts — one of the earliest attempts to find out what city-scale sensing was actually good for, before "smart city" was a marketing term. Chicago's Array of Things took the same instinct public a few years later: 100+ modular nodes on lampposts measuring air quality, noise and pedestrian flow as fully open data, self-described as "a fitness tracker for the city."
Egypt's New Administrative Capital shows what happens when sensor infrastructure is built in from the start rather than retrofitted: a Honeywell-built system links 6,000+ CCTV cameras to a central command centre, with smart columns every 250 metres carrying additional sensing — infrastructure poured into the ground alongside the roads themselves, for a city built for 6.5-7 million residents from scratch.
San Diego's US$30 million CityIQ network — 3,000+ streetlight nodes with cameras, microphones and environmental sensors — was deactivated in September 2020 after the ACLU raised concerns about police access to the footage it had quietly been collecting; it only returned in 2023, under a narrower five-year agreement covering 500 cameras with new operators. It's included here on purpose: a sensor network's technical spec sheet rarely mentions who else can query it later, and that question is often what determines whether a deployment survives in any form.
From a research testbed to a purpose-built capital city to a streetlight network that had to earn its way back after a shutdown, the pattern is the same: sensors are cheap to install and hard to govern well. See the Data & IoT category for how these sensor layers feed into open-data platforms and digital twins.