WASHINGTON — The National Restaurant Association’s Restaurant Performance Index was at 102.2 in December, up 1.6% from November and the highest level in nearly six years.

“Aided by favorable weather conditions in many parts of the country, a solid majority of restaurant operators reported higher same-store sales and customer traffic levels in December,” said Hudson Riehle, senior vice-president of the research and knowledge Group for the N.R.A. “In addition, restaurant operators are solidly optimistic about sales growth in the months ahead, and their outlook for the economy is at its strongest point in nearly a year.

“Coupled with the solid November results, the R.P.I.’s impressive December performance bodes well for continued positive industry momentum in the year ahead. The ripple effect will likely be felt throughout the supply chain as well, with restaurant operators’ plans for capital spending rising to its highest level in more than four years.”

December also represented the third time in the last four months the R.P.I. has been above 100, indicating industry expansion.

In addition, 69% of restaurant operators reported a same-store sales gain in December compared with the previous year, and only 18% saw a same-store sales decline. Fifty-seven per cent of operators reported higher customer traffic levels with 23% seeing a decline. Capital spending is up with 48% of operators saying they made a capital expenditure for equipment, expansion or remodeling during the last three months.

Operators also expect their sales to be higher in the months ahead. Fifty-one per cent of operators expect to have higher sales in six months, up from 41% who said the same thing last month. Operators are also more optimistic about the direction of the overall economy in coming months as 39% of operators said they expect conditions to improve in six months, up from 27% last month. Only 11% of operators said they expect economic conditions to worsen in the next six months.