WASHINGTON — The National Restaurant Association’s Restaurant Performance Index rose to 100.1 in September, a 0.7% increase compared with August and the highest mark the Index has achieved since June.

“The September increase in the Restaurant Performance Index was fueled by improvements in the same-store sales and customer traffic indicators,” said Hudson Riehle, senior vice-president of the Research and Knowledge Group for the N.R.A. “Among the forward-looking indicators, restaurant operators are more optimistic about sales growth in the months ahead, while their outlook for the overall economy remains cloudy.”

Restaurant operators reported stronger same-store sales in September. Fifty per cent of restaurant operators reported a same-store sales gain between September 2010 and September 2011, up from 45% who reported a sales gain in August. In comparison, 34% of operators reported lower same-store sales in September, down slightly from 37% in August.

Restaurant operators also bounced back from a sluggish August performance to report net positive customer traffic levels in September. Forty-three per cent reported higher customer traffic levels between September 2010 and September 2011, while 33% reported a traffic decline. In August, only 34% of operators reported higher customer traffic, while 42% reported a traffic decline.

Many operators who participated in the N.R.A.’s survey said they anticipate sales will grow in the next six months. Thirty-seven per cent said they expect to have higher sales in six months (compared to the same period in the previous year), up from 33% in August, which represented the lowest level in 19 months. By comparison, 19% of restaurant operators expect their sales volume in six months to be lower than it was during the same period in the previous year, down from 23% who reported similarly last month.

The outlook for the overall economy remains mixed, according to the September R.P.I. Twenty-two per cent of restaurant operators said they expect economic conditions to improve in six months, up slightly from 18% who reported similarly last month. Twenty-three per cent of operators said they expect economic conditions to worsen in the next six months, down from 31% who reported similarly in August.