Vibrosight++: City-Scale Sensing Using Existing Retroreflective Signs

Urban environments are vast and complex, with a wide range of facets that city planners, public transit agencies, business owners, citizens, and many other stakeholders might wish to sense. This sensor data can be used by cities to increase mobility, improve operational efficiency, boost quality of life, and become more sustainable. For this reason, the concept of smart cities is gaining traction, but due to high cost, city-scale deployments are rare today except in mature and narrow use cases, such as traffic flow and gunshot detection. This leaves an exceptionally long tail of small, but interesting smart city applications with no viable source of data and without budgets to put them into practice.

The primary reason for such high costs is that today’s smart city sensing paradigm is built around massive deployment of physical sensing infrastructure. One notable example effort is Chicago’s Array of Things project, launched in 2015. Backed by at least $4M in funding, the project has deployed 130 sensing nodes on poles through the city as of January of 2020. In general, these smart city nodes are weather-hardened devices requiring power and connectivity, and cost thousands to install on a pole, not including hardware and maintenance costs, which is many times more expensive. Thus, the central thesis of our research is that to unlock the potential of smart cities, a truly tectonic shift in city-scale sensing is needed to 1) dramatically reduce deployment costs, 2) virtually eliminate maintenance, and 3) generalize across a wide array of sensing applications that can improve cities.

In this research, we present one candidate method for ultra-wide-area sensing that could be applied to the expansive urban landscape and meet our aforementioned three criteria. More specifically, we suggest a novel application of laser vibrometry, used extensively in fields such as industrial quality control, aeronautics, and structural health monitoring. Although typically limited to just a few meters of sensing range, the use of retroreflective markers can boost this to 1km or more. Fortuitously, cities already make extensive use of retroreflective materials for street signs, construction barriers, road studs, license plates, and many other markings. Importantly, the “smarts” (and expense) is limited to the vibrometer itself, with retroreflective markers costing just a few dollars each. We describe how our prototype system can co-opt these existing markers at very long ranges and use them as unpowered accelerometers for use in a wide variety of sensing applications.



Yang Zhang, Sven Mayer, Jesse T. Gonzalez, and Chris Harrison. 2021. Vibrosight++: City-Scale Sensing Using Existing Retroreflective Signs and Markers. In Proceedings of the 2021 CHI Conference on Human Factors in Computing Systems (CHI '21). Association for Computing Machinery, New York, NY, USA, Article 410, 1–14. DOI:

© Chris Harrison