This series has given a taste of how the public transportation industry has addressed myriad crises, technological revolutions and social upheavals in the past 100 years. It has been a great ride, but as the following list of 10 for this century suggests, we might not have seen the biggest and best yet.

1.The sustainability of funding: TEA 21 provided record billions of dollars and budget "firewalls." The compact that forged that law is breaking down because there is not enough money to go around in the long term. Gasoline tax revenues are simply not keeping pace with the need, and without additional revenues — and the political will to raise them — the federal transit program will be unsustainable.

2. Making transit pencil out: The relationship between development and public transport has been an oft-running theme here. What the next 100 years must demonstrate is shorter-term payback, and there is growing evidence it can, particularly with less expensive bus and rail solutions under way. The issue transit agencies must address is whether they are comfortable with that much private participation.

3. Becoming "mobility brokers": A long list of studies have pointed to the need for an oversight body that "brokers" services, whose function is separate from operating organizations. Only a few U.S. cities have tried it, though, because it means a divided role — and divided power.

4. Avoiding the "gold plating" of the system: The steady rise in expenditures in the past decade also carried a much higher price tag, driven by new regulations and the desire of policymakers to use transit as a test bed for transportation technology experiments. Do we really need all this stuff? Wouldn't more frequent, less fancy buses and trains reduce pollution more than deploying fewer, higher-cost vehicles with the cleanest possible advances? Performance standards, better life-cycle costing and more liberalized rules governing service and training contracts would help immensely.

5. Mobility's relevance in a virtual world: The industry's twin raisons d'etre have been sustainable, efficient mobility and equal access. The information age has redefined our need to "be there." Will the public buy what we have to offer when people's sense of community cannot be substituted for a virtual one?

6. The price to pay for speed: To date, maglev advocates have confronted a bitter reality: Neither government nor the private sector has been willing to fund the fastest ground transportation possible. Nor has our society said how much speed they will buy, though. More new roads and airports have been rejected; per capita and adjusted for inflation, we are paying less for faster travel than a generation ago.

7. Bye-bye Buy America?: We will soon approach three decades of federal protection of bus and railcar manufacturers in U.S. transit procurements. If the federal share of capital spending continues to decline, the ability of local agencies to avoid rules such as Buy America increases. Pressure is mounting for tougher intervention; meanwhile, Europe and Canada are lowering their trade barriers. We are headed for an encounter, the outcome of which is uncertain.

8. Getting around when boomers retire: The aging of the Baby Boom generation, which will double the number of people older than 65 by the year 2021, is public transport's huge market opportunity. How the industry can help lead retailers, developers and communities to the graying promised land — and keep them once the much smaller Gen X is in retirement — will be the challenge of the century.

9. Edge cities become urbanized: Despite the ungodly amounts of highway and road building, coupled with the desire to live in less crowded spaces (remember that streetcar lines were the original causes of sprawl), even the outer 'burbs are aging and crowded. Transit must shift its attention to the dying malls, highways-cum-boulevards and other signs of decay/opportunities for rebirth.

10. Balancing a sense of place with shifting mobility: Effective transit-oriented development helps create places, but Americans are conflicted: They yearn for neighborhoods but move frequently. The answer is in the wisdom of our pioneers: They worked with civic leaders, building systems where people wanted to go — and they understood the value of full trains and buses.