10 or more driverless taxis on Vallejo Street in San Francisco blocked traffic on that street for 15 minutes, simply by stopping in the street. All the human drivers in cars on that street were stuck.

The company's explanation makes it clear that these cars are not "autonomous" — they depend on receiving orders via cellular data networks.

The possibility of stopping traffic is, of course, a pain in the neck, and people are right to point out it could be dangerous if an emergency occurs. But I see that as the secondary danger, because improvements in technology will tend to correct it over time.

The biggest danger of automated taxis is that of massive surveillance: tracking the passengers because they must identify themselves to pay, and recognizing people on the street using facial recognition. Improved technology will tend to make these dangers worse.