Does a price on the demand curve represent the highest or lowest price the consumer is willing to pay? My question comes out of the context of the explanation for quotas in the textbook Macroeconomics, 4th edition by Paul Krugman. The explanation is for what could've happened had a quota on cab rides not been placed. In this explanation, the price of $5.50 is apparently the minimum price consumers are willing to offer:


  Looking back at Figure 4-8, you can see that starting at the quota
  limit of 8 million rides, New Yorkers would be willing to pay at least
  $5.50 per ride when 9 million rides are offered. 


In the next paragraph, there is another reference to the price as being the minimum that people were willing to pay:


  The same is true for the next 1 million riders: New Yorkers would be
  willing to pay at least $5 per ride when the quantity of rides is
  increased from 9 to 10 million...


Shouldn't this price be the maximum price that consumers are willing to pay for, instead of the minimum price, as the book suggests? Why would a consumer not want to pay less that $5.50$ dollars per ride, at $5.30$ dollars for example, if the same quantity of rides (9 million) is offered?

I have included a copy of the diagram that is referenced, which I redrew. The x axis is quantity of cab rides per year, in millions of rides. The y axis is the fare price per ride. The green, orange and red lines are the demand, supply and quota restriction lines respectively. Please click to enlarge image.
A distinction must be made between the individual demand curve and the aggregate demand curve.

An individual demand curve (or demand schedule) for a product/service, represents the pairs {price, quantity demanded} that co-optimize the consumer's utility maximization problem (together with all other quantities demanded of the other products that are present in the utility function).
It may appear that then this would be the "maximum" price, but it would be confusing to characterize it like that. Because, if the price changes, what would happen to individual quantity demanded depends on whether the good is normal or not, and in general whether there are "income-effects" or not (not all utility functions allow for income effects, by the way).
Moreover, at the individual level, the concept representing the consumer is quantity demanded, not price: the consumer observes prices and responds with quantity demanded.

Let's move now to the aggregate demand curve. It is simply the sum of individual demand curves for each price. Here we have two alternative approaches:  

a) The more "traditional" one is that all consumers have a non-zero demand schedule for all price levels, each lowering its demand as price increases. Here the previous discussion applies.

b) The alternative but equally "standard" approach is to argue that for each consumer there exists a (possibly different) "threshold" price, above which the individual demands zero quantity. So as price decreases, we imagine that more consumers enter the market with non-zero demand, and this is why as price decreases aggregate quantity demanded increases.
This approach implies that the price level in each price/quantity combination is the minimum ("at least") price for some consumers, while others would be willing to pay more (and so they have contributed positive quantity demanded for higher price levels).

I believe this is the approach taken by the textbook.