From Yahoo News and the Associated Press:
Assuming that each of those fleeing will need to stop for the bathroom every two hours, the report said 810,000 people will be desperately looking for a toilet within hours of the quake, even if half of the public facilities remained intact.
Some might wait 17 hours before finding a toilet, the simulation said.
http://ca.news.yahoo.com/s/capress/japan_toilet_refugees