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.

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“One of the great weaknesses in US privacy law is that we only protect against intrusions into private areas, not public spaces,” said Albert Gidari, director of privacy at Stanford Law School’s Center for Internet and Society.

“Public roads through neighbourhoods, licence plates, pedestrians on public sidewalks etc all are fair game,” he said.

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Stanford’s Mr Gidari said the technology perhaps highlighted one instance where long-existing laws may not have taken into consideration the types of technology on offer today.

“As these systems become more available and new platforms for capturing imagery become cheaper to deploy, we may yet revisit the issue of whether there is a right to be left alone in public, to be obscure or anonymous, or to be free from collection and storage in other’s systems.”

He added: “A few states have laws that prohibit the collection of biometric information – facial recognition would raise the issue.”