Perhaps you could find people that are willing to install these sensors outside their windows. You could run them from a power adapter to get more than 3 days worth of data collection + less hassle from cops.
If you are moving this project into the Tendernob and need a place to put equipment on Pine St, I'd be happy to help out.
Great idea Alex. Crowdsourcing the data collection might be a great route... we'd have to account for calibration and consistency of devices, but I think that could all be overcome.
