Practically the entire change in subway ridership in New York over the last generation or two has come from the off-peak, and the way American cities set their frequency guidelines off-peak amplify small changes in demand, so that a minor setback can lead to collapse and a minor boost can lead to boom.
The good news is that by setting frequency to be high even if it does not look like ridership justifies it, cities can generate a virtuous cycle on the upswing and avoid a vicious one on the downswing. However, it requires the discipline to run good service even in bad times, when bean counters and budget cutters insist on retrenchment. The Chainsaw Al school of management looks appealing in recessions or when ridership is falling, and this is precisely when people who run transit agencies must resist the urge to cut frequency to levels that lead to a positive feedback loop wrecking the system.
Ridership-frequency elasticity
The key to the frequency-ridership spiral is that cutting frequency on transit makes it less useful to passengers, since door-to-door trip times are longer and less reliable. The size of this effect can be measured as the elasticity of ridership with respect to service: if increasing service provision by 1% is demonstrated to raise ridership by e%, we say that the elasticity is e.