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Mass transit taxonomy

LJ inflicts upon me awareness of a light rail vs. subway fight in Toronto, which puzzled me for a long time, since I've thought of e.g. Chicago El and subway as light rail, in contrast to the heavy commuter rail of Metra or Caltrain. And maybe that usage is out there. But I finally found the right readings to make sense of Toronto. And as I like drawing things up in boxes, and some of you may as well, here's my current understanding, plus capacity estimates, minus costs, and with all the rock solid reliability of a tertiary source based on secondary source web pages; if readers know more please correct.

Car: The baseline. What's the capacity of one lane of road? If you recall driving safety, there should be 2 second following distance, and I read that transportation planners assume it. So one point could -- or should -- see no more than 1800 cars go by in one lane in an hour. At 4 people per car that could be 7200 people/hour, but short of urban evacuation or Disneyland that never happens. A commuter lane probably has 2000 people at max capacity, due to occasional carpooling.

Bus: The first resort. Say a bus can carry 60 people. in theory you safely run buses every 10 seconds, giving a capacity of 360*60 or 21,600. But no one does this and you'd be challenged to get people on and off that quickly. London seems to run some buses every two minutes, so 30*60=1800. Not bad.

Streetcar, aka tram or trolley: Basically a bus on rails, with lots of local stops, and no faster. Possibly slower, unable to get around road obstacles. Advantages: train cars tend to be bigger than buses, say 60 seats and 120 standing capacity. It seems easier to add a second car, doubling capacity per driver. And people seem to take to them more than buses, whether for reasons classist ("poor people take buses"), cognitive ("rails! i could take a streetcar") or practical. I don't think I've ever suffered motion sickness, but an old friend couldn't read in cars, let alone buses, but was fine in trains. Buses bump and swerve where trains have smoother rides, so the latter may attact more sensitive people.

Capacity 120*30=3600 seats, 7200 crush.

Light rail, aka light rapid transit: A relatively new niche. Street rail optimized for speed. So fewer stops, with a spacing of 500 meters to a mile. That's pointless if you're stuck in traffic, so you also want right of way: an exclusive lane, and pre-emption of traffic signals. Fewer stops means they can be longer, limited by block size, so say 4 car trains. I got the impression headways tend to be longer, say every 3 minutes, perhaps to not disrupt traffic too much. So 4800 seats, 9600 crush. Advantages: high capacity, high speed, low cost. Disadvantage: where metro avoids traffic and streetcars live in it, light rail displaces it. Society has to say "you can have your door to door travel and private bubble, but you don't get all the road or top speed for your car." And some places may not have room for light rail while leaving road for trucks and emergency vehicles.

Terms are fuzzy: London has a "light rail" which is basically another metro system.

"Light" refers to cost or capacity relative to metro, not physical weight, which be heavier so as to win fights with traffic.

Examples: Boston has some near misses from both sides. Of the four branches of the Green Line, the E is pure streetcar. The B and C are not quite light rail, having lanes but not signals and a lot of stops. The D shoots past LRT, having stops every mile but rail in depressed ditches with roads going over on small bridges, achieving the grade separation of metro for much less work than a subway. Similarly, parts of LA and Chicago metros run in freeway medians, getting surface level grade seperation on the cheap.

Metro, from metropolitan, aka rapid transit: Generally comes in subway and elevated versions. Has full grade separation and minimal traffic risk, allowing higher speed and smaller headway. Not interacting with blocks means trains can be as long as you'll build stations for; 8 or 10 car trains are common, I may have seen 12. With three times the length and half again the frequency, capacity can be 4.5x higher than light rail. 12*60*30=21,600 seats/hour, in the space of one freeway lane.

Downside is that where street rail might cost $30 million/mile, I've often seen a billon per miles for subway. Even at $300 million, for half the cost you could build 5 LRT lines for the same capacity and more spread out access; of course, you'd also be displacing 5 lanes of traffic. Metro seems to shine where you need speed for long distances, or where you simply can't or won't build street rail. The US has mostly worshipped the car, so most of our trains are metro; we tore up any direct competition to cars.

Subways with express lines are great, but given the costs, light rail is my new policy sweetheart.


( 3 comments — Leave a comment )
Jan. 21st, 2011 02:50 pm (UTC)
Light rail isn't always located on the street. I think it's fairly common to have a dedicated light rail right-of-way and big, long-platform stations everywhere except downtown, but when the train gets into downtown it joins the street, runs more slowly, makes more frequent stops, and the stops look more like bus or streetcar stops. See: Denver, Portland.
Jan. 21st, 2011 04:09 pm (UTC)
I didn't get into the whole hybridization issue, which is common. San Francisco and Boston the opposite of what you describe: SF lines and Boston Green run in subways downtown, and emerge outside it to turn into streetcars or the Green Line clade I did describe. Chicago Blue switches among subway, el, and freeway medians; Brown Line has a street level segment with traffic crossing bars, which is more like a light rail interaction.

I was in Portland for a few days, but don't remember the details, apart from it being streetcar downtown. I would think that puts a cap on how long the trains can be outside, unless they break up trains before entering downtown, or have routes that bypass it, or have vague ideas of later expansion. If you're stuck with 2-car streetcars for part of the route, building 6-car-long platforms seems a waste, barring one of those contingencies.

There's also a "light metro" category, light rail with extended subway or elevated elements. I guess having to play well with traffic at any point makes for a one-drop rule, and it being forced into "light rail". I saw light metro snarkily described as combining the moderate capacity of light rail with the exorbitant costs of metro, and decided to leave the stupid out of my initial taxonomy.
Jan. 21st, 2011 08:51 pm (UTC)
In Portland they don't break up the trains before entering downtown, so the few downtown stations take up an entire block. And actually, the only other vehicles allowed in those lanes are buses, so it's still kind of like having a dedicated right-of-way. Portland also has streetcars, entirely separate from the light rail system, so if you remember riding on something with only 2 cars, it might have been one of those.
( 3 comments — Leave a comment )


Damien Sullivan

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