Gasoline prices begin their annual summer slide Published July 4, 2013 - 12:05am By JONATHAN FAHEYAssociated PressNEW YORK ??? Gasoline prices are on a summer slide, giving U.S. drivers a break as they set out for the beach and other vacation spots for the Fourth of July.The national average for a gallon has fallen for 21 days straight and is now below $3.50 for the first time since February.