One common argument for building HSR is that it will help decongest airports, by displacing high-volume short-distance flights. This can result in a permanent reduction in air travel, reducing environmental impact, or a diversion of capacity to longer-distance flights, or perhaps a combination of both. The question is then how much air travel can be diverted.

The main source I’m using for this is the Office of Aviation Analysis’s master table of all lower-48 origin-and-destination city pairs with at least 10 passengers per day (table 6, 3rd quarter of ’11). The data is less than perfect, because passengers connecting from a domestic flight to an international flight count as O&D passengers, but for our purposes it is good enough.

As a first filter, we can see that out of a million passengers per day, 206,000 are flying distance of up to 500 miles, and 390,000 are flying up to 773, the New York-Chicago distance. Those 39% of travelers constitute a much smaller portion of emissions than 39% but a larger portion of planes. Furthermore, not all can be realistically moved to trains: at the upper end of this range, HSR can compete with air but not decimate service the way it can on shorter trips, and on top of that many city pairs are not located on any realistic HSR corridor.

So as a second filter, let us construct a table, by major city (i.e. the top 7 O&D cities minus Las Vegas), of what the total volume of travel is to HSR-viable markets:

City <2.5h <3h <3.5h <4h <4.5h <5h New York (153386) 7.4% 10.7% 15.7% 17.6% 20.6% 32.2% LA Area (132556) 11.6% 26.4% 26.4% 26.4% 26.4% 26.4% Bay Area (103752) 0% 18.1% 18.4% 18.4% 30.5% 33.3% Chicago (103540) 9.5% 16.5% 16.7% 19.9% 22.8% 34.1% Was.-Bal. (97234) 5.4% 16.7% 22.5% 23.2% 29% 31.3% Boston (75329) 8.7% 21.3% 23.3% 26.7% 28.6% 31.8%

Although HSR can get nontrivial mode share against air even if it takes 5 hours, it does not reduce air traffic at this range, but instead induces demand. So although HSR can produce competition for almost a third of the air traffic coming into the largest US cities, it cannot divert as much air traffic. Meaningful diversion occurs at much shorter range, perhaps 3 hours, and even that diversion is incomplete. When the 3-hour Eurostar opened, Paris-London air traffic was permanently halved, from 4.3 million per year before the Chunnel opened to about 2 million after; once the travel time was further reduced to 2:15 with the opening of High Speed 1, it further decreased, to about 1.3 million on the dominant Heathrow/CDG airport pair.

What this means is that for decongesting airports, the meaningful column is the second from the left, for trips up to 3 hours. We immediately see that HSR can only have a small effect on New York, but conversely can do a great deal in Los Angeles. New York is at a further penalty since the hub system ensures it will remain an international gateway, and so traffic between two different cities still needs to pass through.

For New York, the best things that can be done then are to use larger planes on domestic flights, and find relief airports. In Japan, the domestic flights use widebodies, sometimes even 747s, and this has enabled Tokyo-Sapporo to grow to become the world’s highest-capacity air city pair. In the US there are more airlines and the city pairs are less thick, but there is still room for larger planes than 737s and 757s. In the other direction, faster LIRR service could turn Islip into a better relief airport, but it would still have to overcome the stigma of being too far. HSR could also turn Philadelphia into a reasonable option: using the Airport Line and a freight corridor to the west to bypass some of the Wilmington Line’s curves and reduce travel time should be considered as a full build-out option, and would also put PHL about 45 minutes away from New York.

The New York versus Los Angeles difference is not too surprising once we consider where their respective second cities are located. San Francisco is 700 km from Los Angeles, Boston and Washington are 350 km from New York and Philadelphia 150. Elizabeth of CARRD tells me that on LA-SF the current mode split is 50% air, 50% car. The situation in the Northeast is different – making reasonable assumptions on seat occupancy, even on NY-DC and NY-Boston more people take a bus than fly.

Update: Anonymouse in comments brings a good point about the distribution of short-haul travel within airport systems: there is often proportionately more of it at the secondary airports. Providence actually has less short-distance traffic than Boston and Midway is about even with O’Hare, but in California, much more short-distance traffic is at the secondary airports.

The five LA-area airports between them have 27.5% of their domestic traffic within 3-hour radius, but this splits as 21% at LAX, 35% at Long Beach, 37% at Santa Ana, 40% at Ontario, and 63% at Burbank. The three Bay Area airports between them have 19% of their domestic traffic going to LA and a total of 35% within 5-hour train radius, but this splits as 14% and 29% at SFO, 27% and 48% at San Jose, and 35% and 57% at Oakland.

Notes about the table:

1. The transfer penalty is set at 20 minutes, for city pairs that have no reason to ever have a one-seat ride. Both low- and high-speed connecting services are included, including HSR trains running through to the legacy network; I am not proposing new HSR tracks to Green Bay.

2. Instead of making hard alignment decisions, I simply ignored everything that would be controversial. The change in numbers is trivial. For example, neither South Bend nor Fort Wayne is included; both combined have only 2,000 daily air travelers anywhere in the lower 48, and only a handful of dozens to each of the cities in the table.



3. The travel times are full-build, so, for example, the Northeast Corridor is 1:30 Boston-New York and 1:30 New York-Washington, rather than the slightly higher travel times that should be aimed at initially. Average speeds range from 240 to 300 km/h on high-speed lines (higher in the Midwest, South, and flat portions of the West, lower in the Northeast and the Californian mountain crossings), and 100-130 km/h on upgraded legacy lines.

4. For US-Canada travel, we use T-100 data for international flights (data from September 2011). The data quality is poor since small planes are excluded, causing an underestimate in traffic on such markets as New York-Toronto, but conversely many of those flights would be double-counted because international-domestic transfers count twice. We can assume that the two effects (ignoring international flights outside Canada, and counting domestic-international transfers) cancel out, which is equivalent to assuming that exactly half of international travelers connect domestically.

5. The full list of cities included in each entry in the table is:

New York:

-2:30: the Northeast Corridor, Hartford, the Empire Corridor up to Rochester, Pittsburgh, Richmond, Burlington, Montreal.

2:30-3:00: Buffalo, Raleigh, Portland.

3:00-3:30: Toronto, Ottawa, Cleveland, Norfolk, Greensboro.

3:30-4:00: Charlotte, Toledo, Fayetteville, Lynchburg.

4:00-4:30: Greenville (SC), Greenville (NC), Columbus, Detroit, Roanoke, Nantucket, Columbia (SC).

4:30-5:00: Atlanta, Chicago, Dayton, Cincinnati, Wilmington (NC), Savannah.

Los Angeles:

-2:30: Las Vegas, Phoenix, Sacramento.

2:30-3:00: San Francisco, Tucson.

(This is where my exclusion of unrealistic corridors has the most effect. HSR could connect Los Angeles with Portland and Denver in 5 hours, Salt Lake City in 3:30, and El Paso and Albuquerque in 4:30. But the population is too sparse for the overlapping short trips that make comparably long corridors in the eastern half of the US semi-reasonable.)

Bay Area:

-2:30: the entire Central Valley.

2:30-3:00: Los Angeles.

3:00-3:30: Palm Springs.

3:30-4:00: —

4:00-4:30: San Diego, Las Vegas (assuming a Grapevine and Cajon alignment, which is the worst assumption; if the connector is between Victorville and Palmdale, as officially planned, then it’s about 4:00, and if it’s between Mojave and Barstow, it’s 3:45).

4:30-5:00: Phoenix.

Chicago:

-2:30: the corridors to Minneapolis, Detroit/Cleveland, Cincinnati, and St. Louis; Grand Rapids, Louisville, Dayton, Green Bay, Columbus.

2:30-3:00: Nashville, Pittsburgh, Buffalo, Kansas City, Toronto.

3:00-3:30: Chattanooga, Rochester.

3:30-4:00: Atlanta, Harrisburg, Syracuse.

4:00-4:30: Ottawa, Philadelphia.

4:30-5:00: Montreal, Albany, New York.

Washington-Baltimore:

-2:30: the Northeast Corridor up to New York, the Southeast Corridor down to Charlotte, Fayetteville, Norfolk, Lynchburg.

2:30-3:00: Boston, Hartford, Albany, Pittsburgh, Greenville (SC), Greenville (NC), Roanoke, Columbia (SC).

3:00-3:30: Atlanta, Wilmington (NC), Burlington, Cleveland, Savannah.

3:30-4:00: Montreal, Syracuse, Toledo.

4:00-4:30: Charleston, Birmingham, Jacksonville, Detroit, Columbus, Rochester, Chattanooga, Asheville, Portland.

4:30-5:00: Dayton, Cincinnati, Buffalo, Daytona, Ottawa. (Orlando is very close and some alignments put it just under 5 hours, but not all do.)

Boston:

-2:30: the Northeast Corridor down to Philadelphia, the Empire Corridor up to Rochester, Burlington, Montreal, Hartford, Portland.

2:30-3:00: Washington, Buffalo, Harrisburg.

3:00-3:30: Toronto, Ottawa, Erie, Atlantic City.

3:30-4:00: Cleveland, Pittsburgh, Richmond.

4:00-4:30: Raleigh, Toledo.

4:30-5:00: Norfolk, Greensboro, Detroit, Columbus, Dayton.