What is the Economic Rationale for Discounted Early-Bird Parking? Most urban parking structures/lots in the US seem offer so-called "early bird" discounts for parking, where a substantially discounted flat rate applies to customers entering before a certain cutoff time (typically 9 or 10 AM) and exiting before late evening (typically 6 or 7 PM). 

The practice is nearly universal, so there must be some economic advantage to it (unless the various players are locked into some kind of weird Nash equilibrium). 

My best guess is that this is a kind of price discrimination targeting people commuting in to work, but price discrimination usually soaks business customers in order to offer discounts to discretionary customers, and this situation is the opposite, so I'm not sure that's it.

It could be that it's especially valuable to kick out a large number of vehicles by early evening so that a "second crop" of customers can be brought in. But evening rates (at least when no special events are being held nearby) are also typically discounted. 

It could also relate to minimizing turnover costs and traffic congestion. But turnover costs for a parking lot (especially an automated one) seem to be a tiny fraction of the overall cost. 

And it could be the result of a price war between suppliers. Knowledge of a particularly cheap nearby lot would tend to spread quickly among coworkers.

Is there a definitive slam-dunk reason these discounts exist? 

EDIT: Another guess is that commuters are high-knowledge customers and shoppers are low-knowledge customers and so it makes sense that the latter would tend to get charged more.
There seems to be some research on price discounting. For example, this paper studies two forms of dicounting, last-minute discounting and early-bird discounting, in settings with monopoly, duopoly and competitive markets. They propose a model and run some experiments. Some excerpts from this paper:


  There might be many reasons for such a pricing policy [Early-Bird Discounting]. Early bookings for example reduce uncertainty for
  airlines, who might be willing to forgo some profit in order to reduce this
  uncertainty. Introductory pricing in markets with network externalities
  might be another reason. Here we want to investigate intertemporal price
  discrimination between different customer groups, who enter the market
  at diferent points in time. One might think of holiday makers versus
  business travellers in the market for airline tickets. Holiday makers
  usually plan ahead and enter the market for a particular flight quite early,
  while business travellers often cannot plan well in advance. Additionally,
  the reservation price of business travellers is much higher than that of
  holiday makers. A monopolist airline will try to persuade as many low-
  valuation customers as possible to book early, such that it can charge
  higher prices later when the high-valuation customers have entered the
  market.


This is also interesting:


  When customers with lower valuations for the good start
  searching earlier for cheap prices than consumers with higher valuations
  then it is possible to sell to these customers at early-bird discount prices,
  while the high valuation customers who enter later can be charged higher
  prices. The main objective of the monopolist is to get the low-valuation
  customers out of the market before the high-valuation customers arrive.
  The low-valuation customers accept early, as they anticipate that the
  prices will go up over time.


They conclude that:


  In the early-bird scenario competition even seems to enhance the ability
  to price discriminate initially, while with increasing experience this advantage is eroded. [...] The monopolists had much less pricing
  power than predicted by standard theory, while the duopolists - also
  contrary to the prediction - were able to retain at least some pricing
  power. The most striking advantage of competition is the increase in total welfare.


Perhaps not entirely pertinent to the case of parking, this entry in The Economist claims that early-bird tickets are useful in events like concerts to build up a minimum mass of goers:


  T in the Park, a large pop shindig held in Scotland, pioneered the practice of offering discounted “early bird” tickets to build buzz.