Second perhaps only to waterways, road systems have had the greatest impact on the design and physical structure of our cities. The car-centric redesign of the American city that began in the early 20th century was embraced with open arms by urban planners and citizens alike. Yet now, in the early 21st century, its limitations are clear. There is a rapidly growing awareness that simply expanding our roadways won't end congestion and gridlock.

The future, more and more urban transportation experts are coming to believe, lies in mobility-friendly networks in which cars are just one element -- and an ever-shrinking one as we move from a system in which the personally owned vehicle is king and toward a multimodal future of on-demand driverless vehicles, ride-sharing, expanded public transit, greater reliance on human-powered transportation and other alternatives.

How far could such a new mobility paradigm take us? Jerry Weiland, a 30-year veteran of General Motors who now leads the Rocky Mountain Institute's mobility program, believes that, over the long haul, the United States could reduce the number of urban/suburban vehicles on the road by up to 90 percent and in the process redefine cities just as the horseless carriage once did.

Whether or not this scenario plays out, it's clear that cities need a roadmap to guide the next generation of infrastructure investment decisions. Roads and bridges last a long time, and new infrastructure is costly. What should city leaders be thinking about when they look at repositioning their infrastructure for the future? "The first thing cities should understand is that all of the transportation infrastructure is about networks, whether it's bike-share, whether it's light rail, whether it's roads," says Cooper Martin, co-author of a 2015 National League of Cities' report, City of the Future: Technology & Mobility. "One line, one bike-share station, one road doesn't cut it."

Martin illustrates this concept by describing how the original Washington, D.C., bike-share system played out when it was piloted several years ago. The city installed a limited number of stations around the city, then declared the system a failure because few people were using it. But later, when the system was built out on sufficient scale, it flourished. The problem wasn't lack of demand but the need to create a network robust enough to be useful.

For many decades, until its own growth began clogging roads and intruding too heavily on neighborhoods, the individually owned automobile was just such a robust solution. Its success sets a high expectation in our minds for an acceptable replacement. "We have to devise a solution that's 100-percent fail-safe," Weiland says. "The new mobility has to offer people a complete answer, not a partial one. Otherwise you're not going to get rid of your car."

It's safe to say that the best fail-safe alternative solution is a multimodal transportation system in which many options -- bikes, transit, car- and ride-sharing -- are readily available at a moment's notice along the direction of travel. With near real-time information, the traveler can seamlessly shift from one mode to another and choose the one most suited to his or her needs.

So how will our cities get to that seamless, fail-safe, multimodal future? Due to varying population densities, economic resources, legacy infrastructure and resident demands, no two communities will take the same approach. Certainly in rural communities -- and no doubt many suburban ones -- the personally owned car will remain the dominant transportation choice for some time.

But in more urban settings, networked alternative transportation choices are already proving to be very dependable alternatives. With improving integration across transportation modes and seamless payment solutions, their growth is all but secured. Our mobility-friendly, multimodal urban transportation future may be closer that we realize.