It's a sight familiar to anyone who's been to a big event - women queued up outside the loo for miles, while men zip in and out without a fuss.
But a new study has found there's a simple fix - let anyone use any toilet, regardless of gender.
Researchers at Ghent University in Belgium say unisex loos cut overall waiting times by 63 percent - far more than any other combination of separate men's and women's facilities.
The researchers developed six alternative toilet arrangements with separate facilities, but found significant improvements weren't possible until there were twice as many loos for women than men.
"The holy grail, however, is to use unisex toilets. In these mixed toilets layouts, the toilet cabins are available for both sexes and optionally complemented with extra urinals for the men.
"As sharing the toilet capacity across sexes is more efficient, the average waiting time decreases. The available toilet surface can be used most efficiently when an ideally balanced layout with about two cabins per urinal is chosen.
"In this layout, men are still privileged, but to a much lesser extent than in the basic situation. The overall waiting time is reduced 63 percent, which cannot be achieved by any other mixed layout, and definitely not by a separated layout."