Sometimes, we have to think in triangles.
In the transit world, for example, we know that ridership arises from a relationship between urban form (including density and walkability) and the quantity of service provided. For example, if we focus on local-stop transit, the triangle looks like this:
You can think of development and service as inputs, and ridership as the output, but you can rotate the triangle any way you like. For example, if you want to justify an increase in service, you want both some existing ridership success and also evidence, from the development density, that there's more potential. In other worse you want to say "Ridership + Development --> Service" On the other hand, if you want dense development as an output, you want a transit line that feels permanent. That's going to be one that already has high service, achieving high ridership. "Service + Ridership --> Development." (In each case, the right-arrow means "supports" or "helps to generate" or "is one of the crucial necessary conditions for")
I am reminded of this triangle because transit discussions are full of assertions about the relationship of two points on this triangle. Some of these are quite famous. Pushkarev and Zupan, for example, are dutifully cited in every article that deals with this topic. WIth a publication date of 1977, they are one of the oldest must-cite authorities in transit research; it's as though they've passed over into the realm of scripture. Their conclusion? "A net residential density of 15 units/acre supports a service level of 10 minutes or better over a 20-hour span." This is a stated relationship between two of the three points on this triangle, service and development, without reference to the third, ridership. So hidden inside the verb "supports" is an assumption about what level of ridership is acceptable.
There is, to my knowledge, no agreed standard about what level of ridership is acceptable, nor should there be. Every city or agency decides that for themselves. That means that logically there can be no answer to the question "is development pattern and density x sufficient to suppoert service level y?"
In the British-influenced world, of course, there's often a hidden implication that the ridership standard should be "profitability." Certainly, if that's your goal, then you've fixed that variable and can now look more coherently at the relationship between the other two; in other words, it then becomes coherent to ask "what service level should be offered in response to each development type, so as to hit a goal of profitability." (Yes, fare policy plays a part, and that's still another variable, but you can also choose to hold that constant.)
But of course, most of us don't think transit should aim for profitability. A profitability paradigm assumes that government has no public interest in the success or failure of transit, just as it has no interest in the success or failure of Coke Zero. And obviously that's wrong.
So when I hear people either quote or update Pushkarev and Zupan, saying that density x supports what level of service y, I hear pure incoherence. The statement says nothing that's useful to me. What am I missing?
UPDATE: Jeffrey Zupan himself wants to assure us that his work was based on a clear ridership assumption, expressed in the form of targets for cost/passenger and cost per passenger mile. Unfortunately, most people who quote his work forget that part. In doing that, they're implying that the ridership target that Pusharev and Zupan chose is the right one for their own community, even though they don't know what it is!