As soon as our species abandoned the daily grind of Stone Age feast-or-famine, we went straight to town. Çatalhöyük, Eridu and Ur were the London, Mumbai and Tokyo, respectively, before 5000 BCE. Çatalhöyük's maze of dwellings are now nothing more than archeological digs. But some things have remained steadfast through the course of millennia. The city then and now is a place to go to break out of old molds and find opportunities. More than half the world population now lives in cities, many migrating to urban centers from rural hinterlands, a trend that shows no sign of abating. That overpowering demographic means the fate of cities will determine the fate of the planet.

Scientific American asked opinion leaders from government, academia and the social network of our readers to answer a simple question:
What is one innovation (technological or otherwise) that would make any city a substantially more livable place?

A selection of the most inspiring answers are printed in the magazine's September issue. Additional impressive replies, edited for brevity and clarity, also appear below.  

Within cities, there should be clusters of tall buildings, designed to leave most of the ground free to be re-naturalized or left in its natural state, providing an urban park with easy access to the building dwellers. Each building or building cluster would have basic services such as commerce, administrative offices, sports facilities and such. The high- density model would greatly simplify transportation and utility networks, while at the same time providing easy access to the natural world, which would be literally an elevator ride away.
Vítor Pereira, Porto, Portugal

Bioreactors in the Walls
We should place sealed containers of algae-based, photo-bioreactors into the sides of buildings to produce biofuels and sequester carbon; as the algae grows it sucks up CO2 from the surrounding air which can then be stored.
Violeta Roxin, Agence de Developpement et d'Urbanisme, Pays de Montbéliard, France

Brain Training for Buildings
The single most important technological innovation that can make cities in industrial countries livable would be development of affordable "smart building" retrofits—a smart building would monitor the quality of indoor air, mechanical and electrical systems, and self-diagnose any faults. For example, it would self-adjust its indoor environment in anticipation of outdoor weather conditions, and warn the occupants of mold problems. Currently, almost all buildings are only reactive, and a large fraction operate with unrecognized faults. With an innovation of affordable retrofits to make existing buildings smart, consumers can save 20 to 30 percent annually on their electricity bills, as well as lead healthier lives since we spend most of our time indoors.
Ashok Gadgil, Lawrence Berkeley National Laboratory

The Walkable Metropolis
I'm excited by the prospect of new technologies improving our future lives, but I'm still waiting for old technologies to improve my current life. I've never lived in an American city where I can walk to shops and to a pleasant cafe, nor one with a reliable or affordable public transit system. In fact, I'm searching for ways to move to such a city as soon as possible because I loved the lifestyle in my years abroad. Even striving to live close to the places I frequent, they are never found near each other, and none are accessible by anything but a parking lot.
Michelle Bennett, Chattanooga, Tenn.

Socialized Health Clubs
Gymnasiums that are free, staffed and open seven days a week for all people, with special activities for men, women, children and families.
Brenda Piquette, Nainamo, British Columbia, Canada

Bicycle Sharing
A system of bicycles and golf carts shared by groups or the city. One person leaves a bicycle, for instance, then another comes and swipes a smart card to then go where they need. Also, subsidized housing must be integrated with "regular" housing, without any indication of who is who, to disassociate stigma and stereotypes about people who need help.
Cherie Parks, Los Angeles

Right now we have the technology for vehicles to guide themselves, without the need for a driver. GM has developed a vehicle that makes driving optional, with intelligence built into the vehicle and communication with surrounding vehicles. This could be an interim stage in development of a longer term alternative, roads with guidance systems able to interact with vehicles of the future to feed back information on traffic throughout the network and work out the best way to get to the destination with minimum delays, based on the number of travelers and their destinations.

There could be transport backbones like present rail or bus systems, with smaller vehicles that people could call up to take them to the nearest backbone, with the system working out the best way to get to the destination. There would be no need to own a vehicle. Fleets of vehicles could be spread around the network by different suppliers who maintain the vehicles and receive income from fares charged to users. Some would be basic, low-cost units, others special purpose or luxury units at higher cost. Users could book a particular type of unit in advance, or accept the nearest unit. From knowledge of the number of people requiring transport and their destinations, the system could work out the capacity and frequency of service needed to cover the demand. When not in use, units would park themselves and recharge.
David Bainbridge, New South Wales, Australia

Victory Gardens
As an architect in Chicago, one of the most important aspects of future city life for me is urban farming. During World War II, "Victory" gardens produced 40 percent of the country's vegetables, and an overall average of 20 percent of American cities are vacant lots. As transportation costs may become a large part of future food price increases, this aspect of city life, an underexploited resource at present, may assume an important place in the sustainable urban experience of tomorrow.
Bruce Blair, Chicago

Self-Organizing Carpools
Most U.S. cities aren't dense enough to make mass transit practical. Google's self-drive car technology could drastically increase welfare in family wealth, time, environment, safety and convenience. These cars could self-organize carpooling by grouping people with similar itineraries. Carpooling, using hybrid or electric cars, will reduce pollution and so far these cars have shown amazing safety records. Finally, this technology fits with the way we've built our cities and lived our lives (for better or worse) for the past 100 years.
Steven Sandhoff, New York

Billboard Bans
A comprehensive ban on broadcast advertising (e.g. billboards, bus banners, excessive retail signage) in public spaces would greatly improve the livability of any city I have ever seen. Also, we should elevate the city-state as the primary institution of government in replacement of the obsolescent, imperial relic of the so-called nation-state.
Corey Ladas, Seattle, Washington