The difference between the forecast and actual growth is relatively small, given the amount of uncertainty in the assumptions on which travel forecasts are built -- things like projections of future population and job growth, where that growth is expected to occur, and what system of roads and transit will be in place at a given point in the future. Forecasters also take into account factors like travel time between origins and destinations, traffic conditions, and the relative costs of available travel modes.

In 1994, the TPB's forecasters used the latest population and job growth projections from the Metropolitan Washington Council of Governments, developed through the Cooperative Forecasting process. The projections called for population to grow to around 4.6 million -- a 23% increase over the 1990 population -- and for employment to grow to around 3.5 million -- a 44% increase.

In actuality, population exceeded forecasts, growing to an even 5 million -- growth of 33% -- while employment fell short of forecasts, growing to 3.1 million -- a 29% rise. And the distribution of that growth among the region's jurisdictions differed from projections, in some cases significantly. In Loudoun County, for example, population grew to 312,000, compared to a forecast 2010 population of 210,000. In Montgomery County, employment grew to 510,000 jobs, fallings short of a forecast of 670,000.

Distributional differences like these can have a major impact on Metro ridership -- greater population and job growth in outer jurisdictions, without easy access to Metro, can lead to less growth in use of Metro, while growth concentrated more in the regional core and inner suburbs would likely lead to increased ridership.

It's hard for forecasters to see some of the demographic shifts and economic swings that result in these kinds of changes, however. In 1990, forecasters could not have anticipated a burst technology sector bubble in the late 1990s. Nor could they have predicted the terrorist attacks of September 11, 2001, or the global economic recession of the last several years. All of these events have had important and lasting impacts on the Washington region's economy, affecting its population and job market, and ultimately the travel patterns of the people who live and work here.

Other factors have also influenced transit ridership in the region over the last 20 years in ways that forecasters couldn't have predicted. A dramatic expansion of the employer-based Smart Benefits program, which provides commuters with monthly subsidies to pay for transit, goes a long way toward encouraging higher ridership. So does new, previously unplanned infrastructure, like the New York Avenue Metrorail station, which opened in 2004, and the District's Circulator bus service, which began operating in 2005.

On the other hand, trends like growth in biking and walking to work, especially in the regional core and inner suburbs, can lead to reductions in the number of people using Metro.

Some of the changes that affected travel patterns over the last 20 years in the Washington region were anticipated, but some were not. The TPB's travel forecasters seek to reduce uncertainty by updating inputs to the travel model each year, and by regularly refining the model to reflect changes in observed behavior. For example, recent changes to the model reflect observed declines in the amount of daily driving per person -- brought on, in part, by the economic recession, increased telecommuting and online shopping and communications, and increased use of non-motorized modes in higher-density parts of the region.

In the end, travel models produce approximate assessments of the future, taking into account only what is anticipated today and what has been observed in the past. Travel patterns and behaviors can change significantly over time due to circumstances that even the most experienced forecasters cannot foresee.

Related Links

• Presentation to TPB Technical Committee: Evaluation of 1994 Transit Forecasts (9/6/13)