What is the actual capacity of BRT?

This is the second in a series about the ITDP bus rapid transit report for Boston, and the ITDP standard in general.

When proponents of Bus Rapid Transit—

You know what? I need to redefine this. I am a proponent of BRT. But I am a proponent of BRT in context. When the ITDP talks about transit, they only mention BRT. Heavy rail, light rail, commuter rail, these are seen as competition, and need to be denigrated whenever possible. BRT is the solution, anything else is not even worth mentioning. 

This is myopic. Bus rapid transit is a tool, but just a a tool box needs more than just a hammer, transit needs a variety of modes working together depending on a city's existing infrastructure, needs and geography. BRT needs to be used where and when it is appropriate, but it is not a one-size-fits-all solution for every transit need. I've already discussed how BRT is not particularly compatible with narrow streets, and how the cities used as analogs to Boston are anything but. 

—So to begin again:

When propagandists of BRT (yup, I went there, ITDP) talk about the benefits of bus rapid transit, they don't tell the whole story. Their argument is that bus rapid transit has the ability to transport as many people as any other mode (45,000 per hour!), at a fraction of the cost. In very isolated cases, this may be true. However, they don't mention that this is an extreme outlier. The infrastructure required for that number takes up enough space that it is compatible only in urban areas with long, wide thoroughfares with space to build. Without this, capacities are an order of magnitude lower, and BRT is much harder to scale than rail.

Here is what the ITDP shows for capacities in people per direction per hour:

This is somewhere in the neighborhood of being true (it's, shall we say, rosy), but it shows absolute maxima, which for BRT are often attained in conditions which, in most cities, are unworkable. (Let's also set aside the fact that 6000 people per hour on a non-BRT bus system equates to 1 minute headways, that a four-track metro like the 6th Avenue Line in New York runs at a capacity of 60,000 per hour and theoretically could run at 100,000 and that light rail is capable of more than 20,000 passengers per hour in, for example, Calgary. So, it's basically not true then; see below.) The BRT number is from Bogotá, and it is an outlier. The way that Bogotá attains that number is by having the BRT system in the center of a highway with wide stations and two lanes for buses on either side, necessitating about 70 feet of street width. This requires four bus lanes at stations, and the street width to accommodate that something many cities just don't have.

Without this width, BRT carries many fewer people. Bus and rail transit scale in two very different ways. Imagine (or look at the chart to the right) a graph where the X axis is the route, and the Y axis is the width of the corridor or the number of lanes/tracks. Rail scales along the X axis, by adding vehicles to the train, so that going from one car to 10 cars gives ten times the capacity. However, adding a second track (increasing the Y axis) only doubles capacity, there are no similar economies of scale. BRT can only lengthen the vehicle so much; most BRT buses top out around 100 feet (carrying about 160 passengers). However, doubling the number of lanes a BRT uses increases capacity by 10 times (or even a bit more; the most frequent route in Bogotá has 350 vehicles an hour—a bus ever 10 seconds!). So while rail can scale by an order of magnitude within a narrow corridor, BRT scales best in another dimension. However, this requires four lanes of width, plus stations, to have the same increase in capacity.

This becomes an issue when capacity is an issue. For a line transporting 1000 or 2000 people an hour, rail is no better than bus: a single-car light rail train every 8 minutes has about the same capacity as a 60-foot bus every 4 minutes. (This is assuming they have similar signal priority, level boarding and fare collection mechanisms to minimize dwell times and unnecessary stops.) Both these frequencies are show-up-and-go frequencies; the average wait time for a three minute headway versus a six minute one is a negligible 120 seconds, a small percent of total trip time.

But if demand increases, a rail line can easily add capacity while a BRT system can not. Increase demand to 3000 people per hour, and a rail line will handle it fine: a two-car light rail train every seven minutes does the trick. However, a BRT system maxes out around 60 trips per hour, and even at this point, even a minor load imbalance (say, from connecting services) or a traffic light cycle missed (say, to allow pedestrians to cross*) will cause bunching. There are diminishing returns at very low headways as being slightly out of sync can cause bunching and crowding issues. There's a reason the BRT line in Los Angeles (the Orange Line) has four minute headways, and not less. Beyond that, bunching, and accompanying diminishing returns, are inevitable.

[Update: Mexico City has more frequent service, it's just that Google Maps transit doesn't show that. Thanks, Google Maps! And I didn't go in to the GTFS file to see what was going on, and it's a somewhat complex file! So, Mexico's BRT system has higher throughput, especially given their longer buses, maxing out around 12,000 per hour. Of course, with vehicles every minute at-grade, bunching is inevitable as crossing phases have to be a certain length on wide streets, so speed declines. It's certainly faster than minibuses in mixed traffic, which the system replaced.]

Beyond 3000 people per hour? A two-lane bus system has problems; crowding will increase dwell times, and capacity or speed may actually go down. A light rail line will reach this point as well, but will be carrying many more passengers when it does so. Boston and San Francisco run 35 to 40 light rail trains per hour underground, with a capacity of 15,000 passengers per hour (Boston, with some three-car trains, actually has a slightly higher capacity). Calgary runs 27 three-car trains (with plans to increase to four) through downtown at rush hour, at-grade! 27 four-car trains will give it a capacity of 22,000 per hour. (Their system carries more than 300,000 riders per day, higher than Boston or San Francisco.) That's on par with pretty much any BRT system (Bogotá's is over capacity, and they are actively looking to build parallel lines to reduce the demand on the main trunk routes.), but the stations and track only take up about 40 feet of street width, enough for a lane of traffic and wide sidewalks in an 80-foot building-to-building downtown corridor, still narrower than any BRT street in Bogota.

In any case, the chart that the BRT report has should actually look something like this, accounting for typical loads and outliers:

Typical loads are lines such as the Broadway IRT for the four-track metro, the Red Line in Boston for the two-track metro, a single branch of the Green Line for the LRT, and the Orange Line in LA for BRT. I took a guess at the typical throughput of a four-lane BRT; I couldn't find any specific schedule or loading data.

Maximum loads are theoretical maxima. For a four-track metro, this is double a two-track metro (the 6th Avenue line is the busiest trunk line in New York, running about 30 trains per hour with a capacity north of 60,000, but could carry more). For two-track metros, several are in the 40000 range: the Victoria and Central lines in London (33 trains per hour, 1150 passengers per train), and the L train in New York (20 trains per hour, 2200 passengers per train). For BRT, four lane, the number is from Bogota. For light rail, the number is from Calgary, assuming they implement four-car trains as scheduled this year. And for BRT, two lane, the number is from a single-lane, one minute headway system with 100-foot buses (which don't exist in the US).

Two notes:

1. Bogotá's system is an outlier. Most BRT systems carry many fewer passengers, especially the majority of lines which do not have passing lanes at stations to increase their capacity. While light rail can scale dramatically, BRT can not, unless the streets are wide enough. Which, in Boston, they're not.
2. Four-lane BRT is akin to four-track metros in capacity enhancement (a four-track metro can carry, in theory, more than 100,000 passengers per hour). However, a four-track metro is only necessary in very high demand situations; most two-track metros can be scaled to meet demand. Four-lane BRT, however, is necessary even when demand is well below what a typical metro line, or even light rail line, might carry. 

Here's another way to look at capacity. It shows how different transit modes attain capacity: rail by adding vehicles (and, to get very high frequency, extra tracks) and BRT by adding passing lanes and frequency. It also shows a dotted line at 60 trips per hour—a one minute headway. Most systems operate to the left of the dashed line. In the case of rail systems, this is because more capacity is generally not needed. In the case of BRT, however, it is because the system is operating near its maximum. In reality, the lines should curve flatter beyond 30 trips per hour (except for four-lane BRT) as bunching and load imbalances cause diminishing returns.

In any case, it's another way to show that while BRT is a useful tool in the transit toolbox, it has a very finite capacity unless it can be expanded to four lanes (plus stations). If you are trying to design a system which can scale, you either need to have that corridor space available (as is the case in Bogotá), or build a rail line. Without that, bus rapid transit can carry about 2500 passengers per hour, but it can't scale higher.

[ * A note on pedestrians: surface BRT is constrained by the length of crossing traffic light cycles. Even with full signal preemption, a crossing cycle needs to be long enough to clear crossing traffic, and for pedestrians to cross the street. In most cases, a BRT corridor will be wide enough to require 30 seconds of pedestrian crossing time. At 5 or 6 minute headways, this is not a problem; the BRT only requires 10 or 15 seconds every two to three minutes, or so. At three minute headways, it requires 15 seconds every 90 seconds, and at two minutes, 15 seconds every 60, and at a minute, BRT requires half of the signal time. It is likely that buses at this frequency would, at times, be forced to stop because of the length of the pedestrian phase (and to keep cross traffic flowing at all), which would create bunching and crowding problems downstream. Again, most single-lane BRT networks operate at four minute headways, which constrains capacity. Beyond that, they lose signal priority advantages, which constrains speed. In other words, there's a fair argument that for surface transit, a three minute headway may be better than a one minute headway.]