A start-up that lets residents monitor who drives in and out of their neighbourhood was among the companies revealed at a Silicon Valley event on Monday.
Flock's sensor, which it offers for $50 a year per house, logs the number plates of every car that drives into a street and takes a picture. The sensor could eventually provide facial recognition.
Residents of monitored neighbourhoods can opt-out of being tracked - but visitors, or people passing through, cannot.
Flock is backed by Y Combinator, a start-up “incubator” which in the past has funded successes including Dropbox, Reddit and AirBnB.
A privacy expert said he believed the data collection to be legal according to US law, but that the idea could ignite a debate about the "right to be left alone in public”.
