Daily Archives: August 24, 2014

Capacity 101

Another sidebar to an upcoming post on Sepulveda Pass (soon, I promise!).

Revised based on some input from Paul Druce (@ReasonRail) and Alon Levy (@alon_levy).

You often hear opponents of rail transit like Randal O’Toole making preposterous claims about the capacity of bus lanes, like saying they can move over 100,000 passengers (pax) per hour. So here’s a short reference guide to the capacity of different types of infrastructure. We’re going to look at one lane or track in one direction.

Freeway lane (passenger cars): the capacity of a freeway lane is about 2,400 passenger cars per lane per hour (pcplph). Assuming an occupancy of 1.5 people per vehicle, that’s 3,600 pax/hr. If you assume that the cars are full, with 4 people per vehicle, and that driverless cars will allow headways of 1 second, that’s 3,600 pcplph and 14,400 pax/hr. As Alon points out below, a realistic occupancy for commuting is about 1.2 people per vehicle, or 2,880 pax/hr. I’m being a little generous with 2,400 pcplph too, the point being that even with generous assumptions, bus and rail have higher capacity.

Exclusive guideway (bus): a 60-foot articulated bus has a standing load of about 90 people and a crush load of about 120 people. If you assume one minute headways, that’s 5,400 pax/hr standing load and 7,200 pax/hr crush load. If you assume 20 second headways (or maybe more realistically, a three bus platoon every 1 minute) that’s 16,200 pax/hr standing load and 21,600 pax/hr crush load. This is a pretty aggressive assumption for bus operations, and labor costs would be high, but it might be doable with an exclusive ROW and good dispatching.

Exclusive guideway (light rail transit – LRT): LACMTA’s design criteria specify a full load of 164 people and a crush load of 218 people for a light rail vehicle. A reasonable assumption for minimum headway on LRT is about 2 minutes, or 30 trains per hour (tph). Metro specifies a design headway of 100 seconds and an operational headway of 2.5 minutes. With CBTC, 2 minute headways are easily achievable. For three-car trains, like LACMTA runs, that’s 14,760 pax/hr full load and 19,620 pax/hr crush load. Go with four-car trains, and that bumps you up to 19,680 pax/hr full load and 26,160 pax/hr crush load. If you could drive headways down to 90 seconds (about what the slightly dysfunctional MBTA Green Line runs), you could get 40 tph for 26,240 pax/hr full load and 34,880 pax/hr crush load.

Exclusive guideway (heavy rail – metro): LACMTA’s design criteria specify a full load of 180 people and a crush load of 301 people for a heavy rail vehicle. Headway assumptions are the same as for LRT. For six-car trains at two minute headways, that’s 32,400 pax/hr full load and 54,180 pax/hr crush load. For a ten-car train at two minute headways, 54,000 pax/hr full load and 90,300 pax/hr crush load. Get it down to 1.5 minute headways and it’s 72,000 pax/hr full load and 120,400 pax/hr crush load.

Here’s a summary table.

capacity

Note that even at this level, we’re not playing fair between rail, bus, and auto. Rail capacity is effectively limited by the signaling system and other regulations like NFPA 130. Bus and passenger car capacity is limited by station capacity. Berthing a train every two minutes is a breeze. Berthing a bus every 15-20 seconds is very difficult, even with long platforms. Berthing a passenger car every 1.0-1.5 seconds is impossible.

So how do people like O’Toole get outlandish capacities like 110,000 pax/hr for bus, while claiming rail has capacity below 10,000 pax/hr? Easy, posit a bus system that doesn’t actually work, and cleverly sandbag rail.

On the bus side, O’Toole assumes a capacity of 1,100 buses per hour or about one every 3 seconds. That works great as long as no one ever has to stop. You could only operate a bus lane at that volume if it was just a trunk that many bus routes used between their origins and destinations, much the same as how cars use a freeway. Think about it: if you were running 1,100 buses per hour on the 405 and 1,110 buses per hour on the 10, you could never hope to have transfers between the lines. You couldn’t operate transit lines, only point to point services, or lines with no stops or transfers between them on the trunk.

On the rail side, O’Toole assumes 3 minute headways, versus our 2 minute headways. What’s a minute among friends? Well, going from 3 minutes to 2 minutes increases capacity by 50%. If you run with CBTC and get 1.5 minute headways, that’s twice as much capacity as O’Toole calculates. In other words, when headways are low, small differences in headway make a big difference in capacity.

At high passenger volumes, rail is still the best option, offering lower operating costs and better reliability. It’s easier to run trains at 2 minute headways than buses every 15 seconds. For lower passenger volumes, bus is often fine, but remember that, as Jarrett Walker says, the most important things is the quality of the ROW. Few cities need to move as many people as the Lexington Av subway in New York. Start with a high quality ROW, then pick the mode that’s the best combination of cost effectiveness, compatibility with existing systems, and accommodation for future growth.