Special traffic police watch green taxis & buses running on LP gas in Mexico City. Situated in a valley this area suffers from severe air pollution, lead free natural gas is cheaper and cleaner. These vehicles have lower running costs & qualify for tax breaks. © David Hoffman phone 020 8981 5041, fax 020 8980 2041, email lib@hoffmanphotos.com. NUJ & BAPLA recommended terms & conditions apply. Moral rights asserted under Copyright Designs & Patents Act 1988. Full terms and conditions for digital and traditional use available on request. Credit is required. No part of this photo to be stored, reproduced, manipulated or transmitted by any means without permission.