In an unlikely coincidence one Thomas Kreutzer wrote for the TTAC today:

Given the natural perfectionist bent of the average Japanese construction worker and the sweetheart deals the Japanese government often makes with local construction and paving companies, you can only imagine those roads; they go places and do things that no road ever should. Isolated ribbons of silky smooth pavement punch through mountains in a gross display of Japanese tunneling prowess.

This explains everything! But wait!

The Japanese anime series “Initial D” gives a pretty good view of the Japanese street racing scene back then. Local heroes in small highly modified cars gathered along the route wherever the road widened just enough to park. // It was there that the vast majority of cars would gather, their hoods open, while sullen young men in black t-shirts bearing nonsensical English phrases shuffled about or stood in small groups, their hands in their pockets and cheap bad smelling cigarettes hanging from their bottom lips. These were the “hashiriya” or runners [...]

Occasionally I would see an accident but they were always minor. The big crashes happened in the dead of night, long after the lightweights like myself had gone home.

Wow.

UPDATE:

[...] One of my students got a little too far over one time and tore the front wheel completely off his car. I thought it was just a dumb accident until I saw Initial D years later, now I know what he was thinking...