For the next installment of 5 Guests, we've asked five world leaders in their fields to tell us what they think the future holds for our cities and mobility issues. While their contributions make very clear the considerable problems we face, at the same time each reveals a quiet optimism that, potentially at least, we could be standing at the threshold of some very exciting and fundamental changes.

"We can already see the green shoots of 'sustainable mobility' breaking through."
Jonathon Porritt
Founder Director, Forum for the Future

"Low carbon cars alone will not solve today's problems, nor meet tomorrow's needs."
Robin Chase
Founder and CEO, Buzzcar

"We have developed a roadmap for realizing the kind of city we hope for, a quarter-century from now and beyond."
Gordon Gill
Co-founder, Adrian Smith + Gordon Gill Architecture

"The car does not stand alone; it is integral to the buildings and their arrangement and to the street and energy systems."
Richard Register
Founder and President, Ecocity Builders

"When thinking about what comes next for transportation, we must first look at how we got where we are. "
Michael Graham Richard
Editor, treehugger.com