I want to know the answer to the following question:
What feature added in 2007 facilitated bus travel by London' passengers with hearing and vision impairments?
All the background I'm provided with is that "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.". Is that enough to answer the question?