Margaret Calvert on her favourite road signs and working with Banksy

Margaret Calvert was in her early twenties when she worked with Jock Kinneir to develop the clear lettering and pictograms of the UK's road signage.

Now in her 80's, she spoke to BBC Newsnight's Kirtsty Wark about her favourite signs and the appropriation and subversion of her work by artists like Banksy.