For defining both MSAs and CSAs, the Office of Management and Budget specifies (different) minimum commuting thresholds. Obviously the choice of the exact value is somewhat arbitrary. But OMB has to make definitions that use data that the Census Bureau collects for the entire country and must use a uniform, consistent standard.
CSAs involve the combination of MSAs (and Micropolitan Statistical Areas) into a single area. This got me to thinking whether other factors might be considered in judging whether areas should be combined and be considered part of a larger metropolitan area. I would like to offer one suggestion: Combined or shared major transportation infrastructure, specifically commuter rail systems and commercial airports.
Going through the major CSA combinations I discussed in the previous post, New York is connected by commuter rail service to its Connecticut suburbs. San Francisco has commuter rail service down the peninsula to San Jose and beyond. And there is additional service from San Jose extending to the east. The three major cities of the Riverside-San Bernardino-Ontario MSA are served by no less than three commuter rail lines, two to downtown Los Angeles and one to Orange County, a part of the Los Angeles MSA.
For the two large combinations about which I initially expressed some skepticism: Commuter rail lines connect Washington with Baltimore and Boston with Providence.
When a single airport provides all or most commercial airline service to two MSAs, it is reasonable to consider them to be part of a single, larger metropolitan area. Raleigh-Durham International Airport serves its two namesake MSAs, of course, as does Greenville-Spartanburg International. The Piedmont Triad International Airport serves the Greensboro, Winston-Salem, and High Point MSAs, hence the name.
The Provo and Ogden MSAs are combined with Salt Lake City into a single CSA. Provo and Ogden each have an airport providing service to just a handful of destination (3 for Provo, 1 for Ogden). So it seems clear that the Salt Lake City airport provides much of the airline service to those MSAs.
The very largest MSAs and CSAs can be served by multiple airports, so one would not expect a single airport to serve both MSAs in those situations. But there are at least two examples of airports located in one MSA that are also seen as providing significant service to another MSA in the CSA. The first one I’d mention is Baltimore’s airport, Baltimore-Washington International. The name says it all. And I’ve flown into it multiple time when going to Washington.
In the greater Los Angeles area, Ontario International is obviously located in the Riverside-San Bernardino-Ontario MSA. Ontario International is literally owned and operated by Los Angeles World Airports, the authority that is also responsible for LAX and one other airport in the Los Angeles MSA. (As an aside, ownership and control of Ontario International is due to be transferred back to the City of Ontario and San Bernardino County later this year. They fought for the change believing that Los Angeles World Airports was favoring LAX over the Ontario airport.)
Common or shared major transportation infrastructure should not necessarily be the sole basis for determining that two areas should be considered part of a single, larger metropolitan area. But I believe it is a strong indicator.