Weaker sales and customer traffic brought the National Restaurant Association’s Restaurant Performance Index below 100 – barely — for the first time in three months in November.

The RPI stood at 99.9, down 0.8% from October. The index represents industry contraction by numbers less than 100. A recessionary slump produced such numbers from late 2007 until September, when it re-emerged into positive territory.

But despite November’s drop, the association found some optimism among restaurant operators and growing demand among consumers.

“Just one out of three operators report that the economy is the most important challenge facing their business,” said Hudson Riehle, senior vice president of the association’s research and knowledge group. “Our research shows substantial pent-up demand. Two out of five adults say they’re not using restaurants as much as they’d like. When their cash-on-hand position improves, they do allocate some of that to restaurants. But it’s still an extremely frugal consumer.”

The NRA found 37% of operators expect the economy to improve within six months, while 15% said they expect it to worsen. Just 16% expect to increase staffing, while 14% plan to reduce it. The restaurant industry had a net gain of 157,000 jobs in the first 11 months of 2010, Riehle said.

On the down side, just 40% of operators reported a gain in same-store sales in November, down from 51% the month before. And 45% said customer traffic was down, up from 34%.

“Just as 2010 was better than 2009, so 2011 will be better than 2010,” Riehle said. “But there’s no rebound to prosperity. Compared to historical performance, consumers’ confidence and cash on hand remains constrained. An anxious consumer doesn’t spend as freely. There’s still a long way to go.”