As economists track mounting evidence of a recession in the U.S., data released yesterday reveal that a key measure of the market for architectural services, the Architectural Billings Index (ABI), a survey of firms’ billings compiled by the American Institute of Architects (AIA), fell steeply during the month of February—the second tumble in as many months and the largest consecutive decrease in the ABI’s 13-year history.

From its score of 55 in December 2007, the ABI dropped 4.3 points in January, ending the month at 50.7. This was followed in February by an 8.9-point plunge, for a score of 41.8. An ABI number over 50 indicates an increase in billing activity, below 50 represents a decrease. February’s numbers marked the biggest monthly decline since October 2001, when the nation’s economy was last in a recession. Studies suggest that the ABI is a good predictor of construction activity nine months to one year in the future.

The AIA had been anticipating a decline in billing—last year its economists issued a negative forecast for nonresidential business during 2008 and 2009—but the severity of February’s downturn was something of a surprise. “It’s not a one-month thing,” Jennifer Riskus, the AIA’s manager of economic research. “It’s probably going to be with us for a while.”

In comments accompanying the survey, which is sent to 300 mainly commercial firms, architects reported that the credit crunch and broader economic concerns are beginning to affect their bottom line. Commercial construction, in particular, could be in for a prolonged slowdown. While the housing sector is at the heart of the current fiscal crisis, the ABI data confirms that, with few exceptions, design and construction activities are being scaled back across the board.

But there were some signs of resilience, including survey comments from a few architects who claimed to be thriving in the current economic climate. Firms that focus on the institutional sector—which includes schools, hospitals, and government buildings—actually posted an increase in billings during February, with a category-specific ABI score of 54.9, up from January’s 51.7.

Firms focusing on the multi-family residential market came in at 46.6, compared to 55.4 in January, while those in the commercial and industrial sector logged an ABI of just 40.6, significantly down from 54.5 in January. Mixed practices posted a score of 43.9, down from 51.3 in January. On a regional basis, firms based in the Northeast fared best, with a 51.5 score, whereas those based in the South (48.3), West (46.3), and Midwest (42.6) averaged lower billings for the month.

In addition to billings, the AIA also tracks the number of new business inquiries that architects receive during the course of a month. The index of inquiries posted a 54.3 score in February, representing a more than 5-point drop from January, but Riskus says that it’s difficult to extrapolate future billings from this data.

Despite the recent spate of bad news, some observers remain optimistic about the severity of the commercial construction slowdown. “The last really major downturn was in the late 1980s and early 1990s, after a period of overbuilding,” says Robert Fox, a founding partner of Cook+Fox Architects, based in New York City, whose Bank of America Tower in midtown Manhattan is slated to open this May. “Back then there was a huge vacancy rate in office space. Now, there’s low vacancy, even as financial institutions are about to give back space” after the latest Wall Street shakeout.