Established in 1918, the Peak to Peak Highway is the oldest Historic and Scenic Byway in Colorado. Photo: The Colorado River finds its headwaters on the west side of Rocky Mountain National Park. On the north, the Peak to Peak Highway leads to the town of Estes Park, just outside Rocky Mountain National Park. Estes Park, Colorado is one of the state's most popular destinations for families on vacation. It is the main gateway to Rocky Mountain National Park.