There is a "Daily Urine Splash Estimation in the US" section in their paper (I can't believe I looked this up). There equation basically makes these assumptions:
1. 56 million non-residential urinals in the US.
2. average of .22 L per "void" (a void is one pee session)
3. I think how they estimated average usage per urinal was weird and frankly wrong - they estimated each person would have between 3 and 6 "voids" per day, and each urinal would be used by between 1 and 2 people per day. Anyway, in any case that leads to an estimate of each urinal being used between 3 and 12 times per day. I think this estimate is way, way too high, because at the low end 3 X 56 million = 168 million, so on the low end they are estimating that, on average, every male in the US makes at least one public urinal pee (and, on the high end, 4 public urinal pees!)
4. Based on their data they calculate a value of ~1% (0.965%) of pee gets splashed onto the floor.
So you multiply that all together: 56 million * .22 * (3 on the low end, 12 on the high end) * .965% = about 350,000L on the low end, or 1,400,000L on the high end, so they said "on the order of a million liters".
Again, I can't believe I spent time looking this up and writing this comment.