It is exciting to observe the next sleek residential tower breaking ground, bemused though we may be about the rent or cost of a unit. Cities do not, however, derive benefit from private investment alone, and prosperous times can bring about complacency about civic ambitions. What we could use is some of the audacity of action that compelled the city forward at other moments of its history. The paradox is that periods of disinvestment rather than prosperity have tended to produce the city's most ambitious undertakings. Perhaps Imagine Boston 2030 can offer an exercise in planning from strength rather than desperation, "civic chutzpah" that is not the product of hard times.

By the end of 2015, more than 80 major projects were under way, totaling more than $7 billion in construction value. What more might a city wish? Well, the 80 represent solid private enterprise, primarily for well-to-do sectors of the economy; public purpose (beyond growth) is not a primary motive. How will the completion of these, and the start of the next 80, help Boston's broad citizenry? Will the city become more desirable, affordable, equitable by the year 2030, or sooner? Reading about the virtues of a penthouse overlooking the Back Bay, available to rent for $35,000 a month, some anxiety about the future accompanies the sense of pride in how far Boston has come since its last comprehensive plan in 1965.

A steady stream of private investment is essential for any city. But can it be channeled for greater public good? Beyond welcoming development, is our public sector keeping up with its responsibilities, with sufficient policy and planning focused on housing affordability, transit expansion, environmental resilience, education and career training, regional cooperation, and the like? These are the questions that come to mind while scanning the forest of construction cranes on the skyline.

It is well to remember that Boston is the city that quadrupled its landmass by creating developable land from mud flats and unnavigable shoals. An elegant neighborhood was built by filling in the Back Bay of the Charles River, while solving a citywide sewage crisis. If there is a contemporary counterpart, it is not yet the Innovation District. Doubts grow whether it can become so, perceived as soulless and already traffic-congested, despite still empty adjacent acres.

Where is the current effort comparable to the building of America's first subway more than a century ago? How about the dedication required to create an Emerald Necklace, then inventing a commission for conservation and infrastructure management, becoming the basis for the nation's first regional plan? Bostonians invented the idea of a Charles River Reservation, designed the Esplanade, and gradually lined the river's edge with miles of public places and trails. We constructed a massive central artery, recognized it as a problem and ultimately depressed it while widening it. In the process, the city gained nearly 300 acres of parkland, in addition to the Rose Fitzgerald Kennedy Greenway.

These are some of the notable transformations the city has undertaken, matched by a history of technological achievement: from clipper ships to the invention of the telephone to advancing radar research and introducing the microwave oven to bringing forth the computer age to accommodating one of the world's largest concentrations of educational, research, and healthcare institutions.

In contrast, last winter's revelation of the scope of deferred maintenance for the MBTA is making it difficult to move ahead with the rather modest expansion of a Somerville-bound Green Line. Always further away is another necessary transit addition. The long-anticipated "Urban Ring" is key to a city in which not everyone is heading downtown. Realizing it would alleviate substantial pressure on the T by diminishing the all-too-frequent need to use two of its spokes to get to one's destination. Highway engineers long ago figured out the importance of circumferential movement across a contemporary urban territory. Should we not move ahead on a transit equivalent of Route 128? For a culture that no longer sees suburban acreage as necessary for the advancement of technology, is there doubt that a circumferential transit line would benefit Boston as "America's Technology Highway" did for the region a half-century ago? A period of local prosperity, at least as evidenced by all that construction, and with growing interest in urban living, should enable the advance of needed infrastructure such as the transit ring.

Cities as unlikely as Dallas and Phoenix are investing substantially in transit. It would be terrific if Imagine Boston 2030 could promote projects like the Urban Ring, even with funding not immediately apparent. No goal, no gain, to paraphrase our stalwart marathoning community.

With pride we note Boston's 60,000 gain in population over 15 years and expect another 50,000 or more by 2030. How will the newcomers move about, and will they be able to afford to stay? The city's destiny seems entwined with today's millennial generation, admired for shifting away from America's century-long fascination with suburban living. Approaching 2030, today's millennials will be in the thick of career and family nurturing, their needs and preferences likely to alter. It is today's school-age population—not all of whom will become app designers, venture capitalists, or celebrity chefs—who will be making their career and dwelling decisions nearer to 2030. How well we provide them with adequate mobility, sufficient job options, and affordable housing will determine their future allegiance to Boston. The current "return to the city" trend is not permanently assured, especially as the cost of living in Boston spirals upward seemingly exponentially. A clue arrives with the US census reporting that today a higher percentage of millennials lives with their parents than during the depth of the recent recession.

Times do change. Between 1950 and 1960, while local high schoolers were reading a civics textbook titled Surging Cities, Boston shed more than 100,000 residents! By the end of the 1970s, another 150,000 Bostonians would flee. A 1960s column in The Boston Globe described "A hapless backwater, a tumbledown has-been among cities." The surge was out of town.

Amid the un-surging city, this 287-page textbook was intended to develop "among the young citizens of Greater Boston a better understanding of planning problems and encouraging a broader participation in their solution." It is a terrific read not only because of the audience for whom it was intended; but also because today's young citizens would benefit from its civics lessons. Surging Cities is worth rereading for the powerful way in which it advocates urban planning—and public investment—as means to a better urban future.

Faced with a rapidly diminishing tax base and people and jobs heading to the burbs or the Sunbelt, city leaders placed faith in public initiatives: The federal dollars beginning to accompany urban renewal programs. Hindsight questions the wisdom of allocating the majority of those funds to highway expansion and "slum" clearance. But some of the audacity of that period would be useful today.

Evoking the urban renewal era while discussing the present is likely to be misunderstood. There is no desire for a return to those top-down, citizen input–free, neighborhood-eradicating processes. At mid-20th century, the near total absence of private investment necessitated radical action. Boston's leaders hoped public works—like a modern government center for the "Cradle of Liberty"—would reverse the city's long declining fortunes.

With today's better fortunes, why not enlarge possibilities? Let's stop resting on the laurels, or regrets about the costs, of the Big Dig. Let's start imagining—reaching for—the next "Big Dig."

• If, as reported, the shortfall for "fixing" the MBTA is already an unimaginable $5 to $6 billion, let's add a modest 15 percent and decide that a transit line to intercept the "spokes of the hub" must be part of the necessary fix. And if not the Urban Ring, how about at least the Bus Rapid Transit long promised for that huge expanse of Roxbury/Dorchester that has only a few bus lines?
• Let's conceive and argue for a regional planning authority with actual authority. Few of our pressing issues, whether traffic management, housing affordability, climate change preparation, or resource conservation are solvable at the level of individual municipalities.
• Let's develop neighborhood support for greater density, without which accommodating population growth will be difficult for a city of a mere 48 square miles. Given citizens' concern about the impact of density, a "carrot" may be required. How about if neighborhood density increases by some preestablished percentage, property taxes in that neighborhood would be guaranteed to remain stable for a time or be reduced?
• Let's reassess the various restrictions along the Harbor that call for "water-dependent uses." It may be time to acknowledge that the proverbial longshoremen are unlikely to return in droves. For a postindustrial century encouraging "uses attracted to and enhanced by proximity to water" might be a more beneficial policy.
• Let's transform the Emerald Necklace into an Emerald Network for biking, walking, and recreating (as the Livable Streets Alliance campaigns do) so that the strands of a regional open space network reach every neighborhood, giving every citizen ready access to its pleasures.
• Let's invent a tax (tough word, yes) on luxury construction by revisiting and expanding the linkage programs, as Mayor Walsh recently did by increasing developer fees for workforce housing. Beyond affordable housing, linkage fees can be a mechanism for accruing resources to advance other public realm needs not readily provided by the market.

It is today's school-age population—not all of whom will become app designers, venture capitalists, or celebrity chefs—who will be making their career and dwelling decisions nearer to 2030

• Let's strengthen the Boston Redevelopment Authority instead of constantly threatening to dismantle it. Organize it to enable the proper management of today's complex public/private development and fiscal partnerships. This is unlikely to be done better by separating long-range planning from economic development, zon­ing administration, and neighborhood urban design initiatives. Such disaggregation will more likely produce staff redundancy, confusion regarding responsibility, and less cooperation.
• Let's consider how a more imaginative—but predictable—regulatory and zoning framework would incentivize desirable outcomes. Other cities have incorporated innovative ideas such as transfer of development rights, multiple forms of incentive zoning, and new ways of thinking that focus more on the scale of buildings and forms of neighborhoods than on separating uses. Let's give intelligent zoning a try, instead of perpetuating the current system of posturing and bartering.

Some of these may seem difficult, naïve, or utopian, somewhat like founding "a shining city upon a hill," whose citizens, Governor Winthrop hoped, would "bear one another's burdens." Easing burdens on those who wish to come or remain in Boston is this generation's challenge. Still young by comparison to much of the world, Boston is nevertheless one of the world's admired cities. To remain so into its fifth century will require prioritizing the "building up" of the things urban dwellers value and share, our public realm. For true urban well-being, that is as important as ever-appreciating real estate.