A new system of higher fees in national parks is starting to pay off for visitors and park administrators alike. The system allows parks to keep 80 percent of the fees they collect, and parks are using the funds to build trails, control erosion, fix water and sewer lines, restore historic buildings, extend handicap access, and more. Although the National Park Service is getting an extra $150 million a year through the program, it’s not enough to fully address the park system’s maintenance backlog of $4 billion to $8 billion.