London's double-decker buses are already used as mobile billboards around the city, but their interiors deliver ads to smartphones too. Since March this year, Bluetooth-powered beacon technology has been used to send ads to passengers' mobiles, capitalizing on what marketers call "dwell time" — the 17 minutes of doing nothing it takes for the average bus journey in London.

The scheme was created by advertising giant Exterion and app-maker Mapway, and means that users of the latter's Bus Times London app receive contextually aware ads that "capitalize on the consumer’s exact physical context." The ads rolled out for Android users of the app earlier this year, and this week they launch on iOS too.

The program is a mixed bag. Although using technology to track users' location and deliver targeted ads has implications for privacy, the scope of the project is limited — it only works if users are using a specific app, and there are loads of travel apps for London. The scheme also has some benefits too, as the location-aware beacon technology allows the app to deliver real-time travel updates to users. That's handy, especially if you're new to the city.

Proxama, the company that helped install the beacons on London's buses in July last year, says the scheme is a way for advertisers to "start arresting the rise of adblocking on mobile." And they claim the clickthrough rates are "as much as seven times higher" than those for conventional in-app ads. You could interpret that as meaning users are being served more relevant and interesting ads — but for many people, that's probably not worth the cost of being tracked.