Just as Japanese society is more intricate, if less varied, than U.S. society, the topography of settled Japan is more intricate than the U.S.
Where we would say "this area is too broken up by mountains and inlets so we'll build towns elsewhere" the Japanese don't have that option. The result is an amazing number of bridges and tunnels. I have driven through more tunnels in three weeks and 4000 km. here than in my previous 24 years of driving cars. If you're a fan of civil engineer you'll giggle with childlike wonder every 20 miles or so as you come across a new suspension bridge, elevated road, or new tunnel.
