Here's a crucial passage from the book I'm working on, though it may will end up in the next book rather than this one [Human Transit]. The topic is emotive, so I'm trying to be very carefully factual here. I welcome your critiques in comments. If you disagree on a matter of fact, please provide a reference to a source.

In 2009, the then-popular [but now defunct] blog the Infrastructurist asked its readers whether streetcars are better than buses, and why. Readers came up with 36 responses (listed verbatim here) that formed a good summary of popular perceptions about the rail-bus distinction.

Of the 36 reasons, only six refer to an intrinsic difference between bus and rail technologies. All the others fall into two categories, which I’ll call misidentified differences and cultural feedback effects.

Misindentified Differences

In your city, the rail system has lots of differences from the buses, including technological differences. But that doesn’t mean that all these distinctions are true rail-bus distinctions. For example:

Propulsion: electric vs internal combustion. In most North American cities that have both bus and rail, the rail is electric but the buses use internal combustion (diesel, “clean diesel,” or various forms of natural gas). Electric motors have obvious advantages – in emissions, noise, acceleration, and comfort – but none of these are true rail vs. bus differences. Rail can be run by internal combustion, and buses can be electric. If you want to compare your electric rail option with a bus option, compare it to electric trolleybuses. If you want to compare your internal-combustion buses with a rail option, compare them to internal-combustion rail options such as the Diesel Multiple Unit (DMU).

Mixed-flow vs exclusive-lane operation. Transit speed and reliability are mostly a result of how much you stop and what can get in the way. Rail is more often run in exclusive rights of way, but some streetcars run in mixed traffic and some buses run in exclusive lanes. Monorails never get stuck in traffic, but neither do buses in Brisbane, Australia’s busway system. Most city buses can get stuck in traffic, but so can any streetcar, tram, or light rail vehicle that runs in a mixed traffic lane. (A major problem for BRT in North America is that people keep taking junkets to Latin America, where BRT is powerful but the economic context is too different, rather than to Brisbane, where they could see high-end BRT working in a wealthy city.)

Off-board “proof of payment” fare collection vs. “pay the driver” fare collection. Fare-collection style has big psychological effects. “Pay the driver” slows down boarding and is a greater hassle for all concerned. Some rapid transit sytstems (rail and bus) provide paid areas with faregates, eliminating this delay. The other solution is “proof of payment,” which means that you buy a ticket on the platform (or already have a valid ticket) but you only show it if a roving “fare inspector” asks to see it. If you don’t have one, you pay a fine. Rail is more likely to use “proof of payment” than buses, but there are exceptions both ways, and there’s no necessary link between the rail-vs-bus choice and the fare collection system. High-capacity bus systems are beginning to shift to “proof of payment” fare collection to eliminate fare-related boarding delay. UPDATE : San Francisco now uses proof of payment on its entire bus system.

: San Francisco now uses proof of payment on its entire bus system. Frequency and Span. Your whole rail transit system may be frequent, while some your buses aren’t, and in that case, you’ll naturally associate frequency with rail. As we saw here, a good Frequent Network map, which shows both frequent rail and frequent buses, will clear up that confusion. Buses can be very frequent, while some rail services can run infrequently or peak-only. (We usually call those commuter rail.)

Cultural Feedback Effects

A community’s attitudes toward rail and bus technologies can easily affect they way they are operated and presented. In short, people who believe that rail is better than buses will tend to act in ways that make that belief true. For example;

Differences in investment or care. A community that believes that buses are only for poor people, or that rail is the mode of the future, will under-invest in buses as opposed to rail, producing a difference in quality that will reinforce that belief. It may also hold bus operations staff to lower standards than rail staff, and encourage other cultural differences between bus and rail operations that become real for the customer, but are not intrinsic to the bus-rail distinction.

Perceptions of permanence. If you don’t stop to think about it, rails in the street will make a service feel permanent, especially if you’re used to hearing people tell you that rails imply permanence. History clearly shows that rail systems do stop running if their market disappears. True permanence lies in the permanence of the market, and that lies in the pattern of development [See Human Transit Chapter 14].

Perceptions of legibility. The notion that a bus might do something unpredictable and a railcar won’t is also a cultural feedback effect, typically the result of insufficiently clear and compelling information about the bus network. It is quite possible to build bus services with such a high level of investment in infrastructure, such as stops and stations, that the routing is as obvious as a rail line’s would be; the Los Angeles Orange Line bus rapid transit system is a good example.

Regulatory differences. Government regulation often enforces different rules for road transport as opposed to rail transport. These regulations are themselves a kind of cultural feedback, differences in habit and history between agencies that regulate roads and those that regulate rail. By enforcing different standards and safety requirements, these regulations can cause outcomes that amplify the apparent difference between road-based and rail-based transit.

Different potential for mission-creep. If you build a stretch of road for a busway, there’s always a danger that somebody might try to open it to cars. If you don’t trust your government to protect the stated purpose of a facility, this can be a major decision factor. This issue applies, however, to the narrow range of cases in which a road or lane is being built that could be useful to cars but is closed to them. It is not an issue where the proposal is to reallocate existing roadspace from cars to transit, nor when building a higher-end busway whose design makes it useless to cars even if they were allowed on it.

Intrinsic Bus-Rail Differences

When we set aside those two categories and look at the differences that really follow, intrinsically, from the rail-bus distinction, there appear to be seven, and only the first three of them are always to rail’s advantage:

Capacity. Where demand is high, rail can serve that demand at a higher ratio of passengers to on-board staff, which means that once you absorb the (often large) construction cost, you will be able to offer greater capacity for a given operating cost. A transit vehicle that’s too crowded to board doesn’t meet any of our seven desires for useful service, so this point is often decisive in favor of rail.

Ride quality. Ride quality in buses is improving, and guided busways may give buses an even more rail-like feel, but new rail systems will probably always have an advantage with their smoother running surface. Is the smooth ride of rail indispensible to a useful network? This can be a tough question whose answer may vary from one community to another.

Limited energy-efficiency and emissions consequences tied to the difference between tires and steel wheels. Again, the primary factor governing energy-efficiency and emissions is propulsion (electric vs internal combustion), which is not intrinsic to the rail-bus difference. However, there is a small range of differences that arise from the physics of steel-on-steel vs tire-on-road operation, and that favor the former.

Noise from wheel friction. Most noise impacts are due to internal combustion, which either rail or buses may use, so that’s a misidentified difference. Rail transit lines that intersect streets may be required to install noisy crossing signals — a valid response to the extreme weight of commuter rail trains but more controversial as applied to light rail. These regulatory requirements may be cultural feedback effects. But rail has a further noise disadvantage that really is intrinsic: the tight fit between steel wheel and rail causes noisy friction when going around curves, especially when going fast.

Some variable cost differences. Broadly speaking, bus-based projects that use portions of existing roadway will be much cheaper than building rail for those same segments would be. Beyond that, costs for bus vs. rail projects can be hard to compare. Capital costs for rail include vehicles, while a busway is sometimes run with an existing bus fleet. Certain bus-rail comparisons in certain corridors may turn up significant differences in operating cost that may be valid in that situation, but need to be checked carefully to ensure that they assume the same factors on both sides.

Maneuverability around obstacles is a specific issue for rail in mixed traffic, usually light rail or streetcars. In mixed traffic, minor obstructions routinely occur in a lane, especially if the lane is adjacent to on-street parking. People stop in the lane to make deliveries, get into and out of taxis, and parallel-park. Accidents and breakdowns happen. If these events block a streetcar, the streetcar is stuck. A bus, in the same situation, can often go around the obstruction and continue.

Ability to extend existing infrastructure. If you’ve already built rail on a large portion the length of a travel corridor, it may be logical to build rail on the rest, so as not to create a technologically required connection. On the other hand, busways can often eliminate extra connections because buses can run through the busway but then flow out onto ordinary streets. In each case, an advantage goes to the technology that makes better use of the infrastructure that already exists, whether road or rail.

Of course, in a particular transit debate, you may not have all of the choices that I’ve articulated here. Still, it’s important to remember that most of the things you hear about why rail is better than buses are not true in the abstract, as facts of geometry or physics that follow from intrinsic differences between roads and rails.

It may very well be that rail is culturally better than buses in your city, in which case all you’re really saying is that people in your city think rail is better than buses and will therefore tend to act in ways that make that true. If you’re interested in appealing to your current population, and motivating them to make investment decisions based on their current perceptions about the benefits of rail, that may even be a good reason to build rail even if you don’t need its intrinsic benefits.

But if you’re thinking in longer-range terms, don’t forget: Attitudes, assumptions and perceptions will change over time. Physics and geometry won’t.

UPDATE! See endnotes for this post here!