(Portland, OR) -- Gas prices declined slightly over the past week. Following Labor Day, Triple-A reports the national average declined a penny to three-81 a gallon. Oregon's average declined two cents to four-74. Grants Pass had the highest average at four-94 and Corvallis has the lowest at four-50. The average in Portland was four-83. While demand for gasoline this summer was below average, vacation travel helped boost prices ahead of the holiday weekend.