Pre-crastination: Time deadlines increase walking speeds even when not constraining

In many circumstances, humans walk in a manner that approximately minimizes energy cost. Here, we performed human subject experiments to examine how having a time constraint affects the speeds at which humans walk. First, we measured subjects preferred walking speeds to travel a given distance in the absence of any time constraints. Then, we asked subjects to travel the same distance under different time constraints. That is, they had to travel the given distance within the time duration provided - they can arrive early, but not late. Under these constraints, subjects systematically arrived earlier than necessary. Surprisingly, even when the time duration provided was large enough to walk at their unconstrained preferred speeds, subjects walked systematically faster than their unconstrained preferred speed. We propose that this faster-than-energy optimal speeds may be due to human uncertainty in time estimation. We show that a model assuming that humans perform stochastic optimal feedback control to arrive on time with high probability while minimizing expected energy costs predicts walking speeds higher than energy optimal, as observed in experiment.