"We are curious whether this new technology could infringe on the privacy of the average American, " says the letter from the Congressional privacy caucus which, in the past, has quizzed many tech companies on what they do with the data they gather.

BBC: US politicians quiz Google on Glass privacy

Critics claim that road pricing would merely displace traffic on to side roads, and that it would infringe privacy (because drivers' movements are recorded).

ECONOMIST: Jam today, road pricing tomorrow