Gas prices heading into the Memorial Day weekend hit a four-year low as millions of Americans prepare to hit the road. When adjusting for inflation, the average U.S. retail gasoline prices going into Memorial Day weekend are 14% lower than last year, in large part because of falling crude oil prices On Monday, for instance, the retail price of regular-grade gasoline in the U.S. averaged $3.17 per gallon, 11% lower than a year ago.  Local gas prices which were under $3 a gallon a week ago, are now mostly above the $3 mark.