London:

London's bus network is one of the largest in the world, running 24 hours a day, with about 8,500 buses, more than 700 bus routes and around 19,500 bus stops. In 2013, the network had more than 2 billion commuter trips per annum, more than the Underground. Around 850 million is taken in revenue each year. London has the largest wheelchair accessible network in the world and, from the 3rd quarter of 2007, became more accessible to hearing and visually impaired passengers as audio-visual announcements were introduced. The distinctive red double-decker buses are an internationally recognised trademark of London transport along with black cabs and the Tube.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What feature added in 2007 facilitated bus travel by London' passengers with hearing and vision impairments?
audio-visual announcements