The battle in big cities continues: how do you keep cars out to cut congestion and reduce pollution? Chris Ledgard visits Paris and Barcelona to explore two different approaches. In Paris, the mayor's office wants to ban the most polluting cars, and coloured stickers are being introduced to help the authorities determine which vehicles can enter the city centre. Meanwhile, more and more Paris residents are turning to the electric car-sharing scheme, Autolib. We hear how it works. In Barcelona, urban ecologists are adapting the famous grid system designed by Ildefons Cerda to create 'superblocks' - large traffic-free spaces across the city where the sound of traffic is only distantly heard. Chris talks to the scheme's inventor, Salvador Rueda, and hears about his vision for Spain's second biggest city.

Producer: Chris Ledgard.