Junction of highway # 16a NOTE, highway #244 ends at this point. South to Custer State Park. North to community of Keystone, South Dakota. For travel south via SD. highway # 16a. See Milebymile .com - Road Map Highway Travel Guide SD. #16a - Black Hills National Forest - Custer State Park To City of Custer, SD for driving directions. THE BLACK HILLS NATIONAL FOREST - Junction of Highway # 385 to Mount Rushmore National Monument Black Hills National Forest stretches for 1.2 million acres, offering outdoor adventure like hiking, biking and camping amidst stunning scenery. Find grassy mountain meadows, towering granite peaks, deep canyons, cascading trout streams and clear, clean lakes.