Between 2005 and 2011, the Washington region's population swelled by nearly 350,000 people to around 5.1 million -- an increase of 7.3%. But during the same time, total daily driving on area roadways saw almost no change, holding more or less steady at around 110 million vehicle-miles a day.
What that means is that average daily driving per person in the region actually fell, from 22.9 miles a day in 2005 to 21.5 miles a day in 2011, reversing a decades-long trend of ever-increasing driving.
That reversal was quantified in a recent Transportation Planning Board analysis of travel data from the Maryland, Virginia, and District departments of transportation. The TPB conducted the analysis as part of an effort to update the computer models it uses to predict future travel patterns based in part on existing patterns.
The analysis found that average daily driving per person fell steadily over the six-year period, and fell in all parts of the region.
The biggest declines occurred in the region's outer suburban jurisdictions -- Frederick, Charles, Loudoun, and Prince William Counties -- where the daily averages fell from 24.6 miles a day in 2005 to 22 miles a day in 2011 -- a 10.6% drop.
The next biggest declines came in the core jurisdictions of Arlington, Alexandria, and the District of Columbia, slipping 5.6% to 15.1 miles a day.
The smallest change, as a percentage, came in the region's inner suburban jurisdictions of Fairfax, Montgomery, and Prince George's Counties, where daily driving fell about 4.5% to 23.4 miles a day.
The downward trends in all parts of the region reflect a similar trend occurring at the national level, where total annual driving per person peaked in 2004 and fell 6.7% to just over 9,400 miles per year by 2012.
In the past, analysts have attributed such changes mainly to changes in overall economic conditions: weaker economies lead to short-term declines in driving; stronger economies lead to increases. However, the most recent decline -- which has continued longer than any other in history -- began before the current economic slowdown got its start in 2007.
This suggests that other forces are at play, like heightened sensitivity to fuel price volatility, more widespread use of e-commerce and electronic communications, and increased telecommuting. Shifting preferences toward less travel by car, especially among younger generations, might also be contributing to the change.
In response to these findings, the TPB recently updated its travel forecasting models to reflect the changing relationship between population growth and driving. As part of its update, the TPB also revised its estimates of the share of non-motorized trips made in dense housing and job centers in the region, based on new evidence, gathered through the TPB's geographically-focused household travel surveys, that these shares are higher than previously thought.
Together, these changes have led to predictions of total driving in 2040 that are about 4% lower than last year's forecast, which has important implications. The TPB used the updated model recently to perform the federally-required air quality analysis of the 2013 update to the region's long-range transportation plan, and predicted that emissions of a variety of pollutants will be lower in future years than previous analyses had shown.
Whether the downward trend in driving per person observed over the last several years continues, especially after the national economy recovers, remains to be seen. Through its various data collection and analysis efforts, the TPB will continue to monitor travel patterns, adjust its forecasts and planning activities accordingly, and assess the underlying trends that could affect transportation in the region for decades to come.