Thankfully I was able to use openstreemap to see a side street getting into the town (google maps failed to show it) and talk a traffic guard into letting us take it. From there, we got to the festival within minutes when the normal road was not moving and probably backed up for another 30mn or more. Advise to others: don't go on weekends, the fireworkds were not exactly special and not worth the traffic jam at all. A weekday without traffic is likely a better idea. The festival, though was fun and pretty, too bad it was almost a 4H drive (return) from Niseko, and the bad traffic jam...