If "Goldilocks and the Three Bears" were written today, bike-share stations would play the role of the porridge. A station that's too full is a bad thing, because that means riders can't return a bike there. A station that's too empty is also a bad thing, because that means potential riders can't rent from there. To keep members happy, you need to get the number of bikes at a station just right.
Operators know this as the "rebalancing" problem, and it's not nearly as easy to resolve as it might seem. On the contrary, some of the world's top mathematicians and computer scientists are addressing the challenge right now. In this week's issue of Science, Vienna correspondent Chelsea Wald reports that as many as 30 researchers are devoting serious time to rebalancing—some in collaboration with bike-share operators in major cities.