TPB News

Accuracy of Growth Forecasts Made 20 Years Ago Varied From Jurisdiction to Jurisdiction

Jan 7, 2013

When local planners gathered more than 20 years ago to forecast how many people would be living and working in the Washington region in 2010, they underestimated how much the population would swell and were optimistic when it came to how many new jobs there would be.

Between 1990 and 2010, population grew by a little more than 1.2 million people -- from 3.8 million to an even 5 million -- an increase of around 33%. Local planners participating in the Metropolitan Washington Council of Governments' Cooperative Forecasting process had predicted adding only about 850,000 people, or 23%, to the population, bringing the total to 4.6 million.

On the job front, employment grew from around 2.4 million to 3.1 million, an addition of 690,000 jobs, or about 29%. But the Cooperative Forecasts had called for more than a million new jobs, or growth of almost 44%.

So what happened? Many things can make forecasting future growth difficult, among them unexpected demographic shifts and economic swings.

Planners in 1990 could not have foreseen, for example, the effects of 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.

Recently, the Transportation Planning Board undertook a retrospective study of the 1990 forecasts to see how accurately they predicted actual growth, which provides a useful context for using today's forecasts of growth out to 2040.

In addition to showing higher-than-expected population growth and lower-than-expected job growth, the study found that the accuracy of the forecasts varied from jurisdiction to jurisdiction, sometimes significantly so.

Population growth in the region's three most populous counties -- Fairfax, Montgomery, and Prince George's, which together are home to almost 60% of the region's population -- exceeded the forecasts by 237,000 people in total. In Montgomery County, twice as much population growth happened as planners had forecast.

Elsewhere in the region, Prince William County and Loudoun County both underestimated population growth, too. In Loudoun County, planners had forecast more than a doubling of the county's population, but by 2010 it had more than tripled.

Only four of the ten jurisdictions' forecasts came within 5% of the actual 2010 population, one of which was the District of Columbia, where population actually fell slightly.

On the employment side, a majority of jurisdictions overestimated job growth, especially Montgomery and Prince George's counties and the District of Columbia, which all saw less than half of the job growth they expected. Prince George's County saw less than 20% of its forecast job growth.

On the western side of the region, however, Fairfax, Loudoun, and Prince William counties all experienced higher-than-expected job growth. Loudoun County far exceeded expectations, more than quadrupling compared to a forecast of tripled growth and adding more jobs than any other county in the region except Fairfax, which itself added more jobs than all of the Maryland counties and the District of Columbia combined.

These comparisons between actual and forecast growth have important implications for transportation planners and the TPB. Recently, the TPB performed a detailed analysis of the latest update to the region's constrained long-range transportation plan, or CLRP, in an effort to paint a picture of travel conditions in the region through the year 2040.

Among other things, the analysis of the 2012 CLRP showed a 78% increase in the number of lane-miles of congested highway during the morning peak period by 2040 and that four out of five Metrorail lines to and through the regional core will be at or above capacity, compared to just one today. The results also showed significant losses in job accessibility by car for people living on the eastern side of the region.

This picture of the future relies heavily on forecasts of population and job growth, since growth is the main driver of demand on the region's transportation system. The forecasts for 2040 may turn out to be no more accurate than those made for 2010, but they remain the "best estimate" of the local and regional planners that develop them. And periodic updates of the forecasts provide opportunities for incorporating new information as it becomes available.

The Council of Governments' Cooperative Forecasts remain an important, if imperfect, input into the predictions of future travel patterns. These predictions help planners and decision-makers evaluate the impacts of alternative growth scenarios and major transportation investments in their effort to find those strategies that have the best chance of improving our transportation future.

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