WASHINGTON — The National Restaurant Association’s Restaurant Performance Index improved in September, the first gain in the past five months.

The increased outlook indicated by the index, which stood at 100.3 in September, was driven by growing optimism among restaurant operators as well as improving same-store sales and customer traffic levels. This level was up 0.8% from August.

“The R.P.I.’s solid gain in September was the result of broad-based improvements among both the current situation and forward-looking indicators,” said Hudson Riehle, senior vice-president of the research and knowledge group for the National Restaurant Association. “Restaurant operators reported positive same-store sales and customer traffic levels for the first time in six months, which propelled the R.P.I.’s Current Situation Index to its highest level in nearly three years.

“In addition, restaurant operators are more optimistic about sales growth in the months ahead, while their outlook for the economy rose to its strongest level in five months.”

During the month, 44% of restaurant operators reported a same-stores sales gain, up from 38% who had reported higher sales in August. In addition, 38% of operators reported an increase in customer traffic between September 2009 and September 2010 while 37% of operators reported a traffic decline.

Restaurant operators are also more optimistic about improving sales in the coming months with 43% of restaurant operators expect to have higher sales in six months, up from 38% last month.