Softer same-store sales and reduced customer traffic caused a decline in an index that measures the optimism of restaurant operators.

The National Restaurant Association’s Performance Index was 99.9 in February, down 0.8% from January, according to a news release from the Washington, D.C.-based associacion. Measuring both current sales and future expectations, the performance index tracks the health and outlook for restaurants. February was the fourth time in five months that the index was below 100.

Hudson Riehle, senior vice president of the association’s research group, said in the release that higher gasoline prices and the effect of a payroll tax hike were factors in softer sales. Sales and traffic comparisons were more difficult due to the extra day in February 2012 as a result of the leap year, he said in the release.

“Despite the sales and traffic declines in February, restaurant operators remain generally optimistic about business conditions in the months ahead, which suggests they feel the setbacks will be temporary,” Riehle said in the release.

The Current Situation Index, which accounts for trends in same-store sales, traffic, labor and capital expenditures, was 98.3 in February. That is down 1.4% from January, and below 100 for the sixth straight month, according to the release.

Customer traffic in February was up for just 24% of operators, and 53% said traffic was lower.

The six-month outlook score for the restaurant business stood at 101.4 in February, showing general optimism about business conditions in the months ahead.