Human Transit

does suburban local service get cars off the road? (updated)

See new updates at the end, based on comments to May 4.

Ricky Leong in the Calgary Sun on why Calgary should spend more money serving far-flung suburbs:

Here’s the thing about providing transit to our far-flung suburban neighbourhoods. The benefits of new transit services will be felt by people everywhere in the city, not just those residents who choose to take the bus and train.

Fewer cars on the road should lead to better driving and cycling conditions for everyone, everywhere, for example.

Suburbanites choosing transit over private vehicles should also mean less wear and tear on roads, so the savings from reduced maintenance would be a net benefit to the entire city.

I'm not trying to embarrass Leung, because the error here is extremely common in journalism, as it is in public perceptions.  Last November Lisa Margonelli built a long New York Times article around the same mistake.

Buses circulating in low density suburban areas (as opposed to express to Park-and-Rides) can serve many valid purposes, but getting cars off the road generally is not one of them.

The universal fact that these people are missing is that buses circulating in low-density areas -- especially where street networks require transit to thread slow and complex labyrinths -- is a predictably low-ridership service.  Outside of a bit of rush-hour activity and sometimes a surge at school bell times, this is the fact of life about local service in low-density suburbs.  If you don't believe me, ask for your own transit agency's stop-level ridership data and compare it to the density and walkability of the area around each stop.  Apart from anomalies created by special land uses, you will find it is much higher in more urban parts of your community where these conditions are more favorable.

In 20 years of looking at  detailed transit data, I've never seen a local suburban bus route whose performance (ridership per unit of service cost) was anywhere near that of a frequent urban bus route operating in area whose layout is favorable to transit. 

("Favorable to transit" means (a) higher density, (b) gridded local streets that provide easy direct access to stops for pedestrians, (c) a safe and pleasant pedestrian environment, (d) a mix of land uses and (e) simple linear paths, usually arterials, where transit can operate efficiently and effectively.  These features tend to be found in older inner cities, but they can be replicated and in some cases even retrofitted.)

Indeed, because transit's market is mostly the area within walking distance of a stop, low density and obstructed street patterns are the very definition of a poor transit market that will yield much lower ridership on investment, and that therefore justifies a poor level of service. Poor ridership out of low density isn't an empirical fact; it comes close to being a mathematical fact, because in most cases density around a stop is the size of that stop's market.

To claim that this predictably low-ridership service will result in "fewer cars on the road" is thus a geometrically incoherent claim.  "Fewer cars on the road" is the result of people riding transit, not transit existing.  So a service that is guaranteed to generate few riders per unit of cost will is guaranteed to get fewer cars off the road per unit of cost, compared to one that is guaranteed to generate many ridership per unit of cost -- such as a line in transit-favorable geography or a rapid transit corridor.

The only high-ridership form of public transit that can serve low-density suburbs with obstructive street patterns is based on Park-and-Ride.  Where land values permit, abundant park-and-ride and fast radial services can get cars off the road  efficiently.  Radial rapid transit (bus or rail) is also good because new transit-favorable neighborhoods can often be built later around its stations.   Park-and-Ride needs to be understood broadly as also accommodating dropoffs, Bike-and-Ride, etc.  But the key thing it does not require is for a bus to actually drive around inside a labyrinth of suburban local streets.

If a transit agency's objective is to get cars off the road, then like any business you start by focusing on your competition's weaknesses.  The car is least convenient in areas of high density and good walkability, and geometrically these also provide the the highest ridership per unit of investment.  The one other area is the suburban commute corridor -- the freeway into the city -- where congestion during peak periods makes the car a weak competitor.  That's why peak commute services -- to Park-and-Ride, not to people's front doors -- is also a high-ridership prospect, and one that gets cars off the road efficiently.

Again, there's nothing wrong with running bus routes around in low-density suburbs.  But it is a low-ridership proposition and therefore can't be justified by many sustainability outcomes; it's justification has to lie in social needs and perceptions of equity among neighborhoods.  If your city really wants to get cars off the road, however, it's not a good way to serve that purpose.

This kind of confusion is why elected officials should be asked to think more clearly about how they want to balance the conflict between ridership-related goals (including lower subsidy, fewer cars on the road, and resulting sustainability outcomes) or coverage-related goals (including lifeline access and "equity" across all arts of a community.  Both goals are noble, but don't pretend to be doing one if you're really doing the other. 

Better still, ask elected boards to adopt policies about how much of the budget should be spent pursuing ridership -- which means running services where high ridership is the predictable outcome -- and how much should be spent pursuing coverage, i.e. distributing service everywhere regardless of low ridership.  The Reno area's transit board did this in 2005, as the result of work I did with them, and as a result, this whole conversation is much clearer, less personal, and more clearly tied to the community's actual values.

For a more detailed exploration of this fundamental issue, see Chapter 10 of Human Transit.

UPDATES:  Comments on this one are often critical, but what I see in them is that:

• People have different definitions of "suburban".  It borders on being an unhelpful word but we don't have a good word for it.  What I meant was: landscapes whose geography is intrnsically hostile to transit, specifically the absence of all or most of the following:  (a) relatively high density, (b) gridded local streets that provide easy direct access to stops for pedestrians, (c) a safe and pleasant pedestrian environment, (d) a mix of land uses and (e) simple linear paths, usually arterials, where transit can operate efficiently and effectively.
  
  
• As in almost everything, we're talking about a spectrum rather than a pair of boxes.  When I talk about dividing a budget between ridership and coverage, it does sound like I'm dividing the world into two boxes.  In fact, many services are in the middle, contributing partly to a high-ridership outcome but with segments or periods that don't.  See Chapter 3 of Human Transit for more on this language problem ...
  
  
• Ridership can be good by the standards of a local subarea and bad by citywide standards, if the entire subarea is less transit-friendly in the terms outlined above.  Another way this issue is sometimes handled is to divide the region firmly into subarea, put budget walls between the services for each subarea, and then assess ridership by each subarea's standards.  This used to be how it worked in Seattle's King County Metro, and the effect was that a line in the suburbs with a certain performance could be called successful while a line with the same performance closer-in to the city would be judged a failure.  Not everyone could follow the "logic" of that and the agency has now let go of that rigidity. 
• I am making a point about geometry, not about culture or values or psychology or ethics.  I'm certainly not telling people where they should live, only about the consequences of those choices.  But concealing or ignoring the facts of geometry leads to policy based on denial, and that leads us in circles rather than forward.

This isn't an easy topic.  Many of the terms are emotive.  But there's an important fact here.  Again, see Chapters 9 and 10 of Human Transit for much more thorough discussion.

shared housing and shared transportation (guest post by alfred twu)

Alfred Twu lives in the Bay Area and is a long time transit rider.  He has studied architecture and business and is also an active participant of the cooperative movement, having worked in artist, food, and housing co-ops. He is also an illustrator for my forthcoming book Human Transit.

Also known as cooperative living, co-ops, communes, intentional communities, or living with roommates, shared housing creates challenges and opportunities for transit service.  It can increase density without zoning changes or construction.  However, good transit service needs to already exist for this to happen.  As such, shared housing's greatest potential is in increasing utilization of under-zoned but well served neighborhoods. 

What is shared housing?

Shared housing denotes a group of unrelated people live in a single dwelling unit.  Kitchens, bathrooms, and other living areas are shared, bedrooms may be shared or single occupancy.  In the Bay Area, this model is known as a co-op or cooperative.  Note that the word co-op has an entirely different meaning on the East Coast.

Shared housing has a long history in the Bay Area, dating back to boarding houses.  The modern Bay Area cooperative housing movement began in 1933 with the founding of the Berkeley Student Cooperative. 

Although some structures are built with shared housing in mind, usually the building is simply a repurposed large house.  For example, Ridge House, a 38 person student co-op, used to be a mansion, while Cooperative Roots, pictured below, houses around 20 people in two adjacent single family houses.




What does this have to do with transit?

Let's look at two neighborhoods: one dense, and one sprawling.  The denser neighborhood will usually have better transit service.

Now, if zoning ordinances prevent additional housing from being built in Sprawlville, that does not mean it's the end of the story.  While zoning typically concerns itself with units per acre, the density that matters for transit service is population per acre, or more specifically, commuters per acre.

This is where shared housing comes in.  When a neighborhood of single family houses goes from having one or two working adults per house, to having 4 to 8 working adults, as far as transit is concerned, it's a high density neighborhood.




The catch to this though, is that this can only happen where there is already good transit service.  When a group of people share a house, they'll all want easy access to their jobs, which may be in different directions.  Not all of them will take transit either - some of them will need to drive to their jobs, some people prefer to bike.  This is why shared housing works so well with college students - everyone is going to the same place for their "job", so only one frequent route is necessary.

Case Study: Ashby BART  (San Francisco Bay Area)

A number of my friends who used to live in shared housing as students have formed their own communities after graduation.  Most are now in their mid 20s to early 30s.  Some work office jobs with traditional 9-5 hours, others work retail jobs with varying hours.  Over the last few years, about 30 communities of 4 to 18 people have been formed.  Most cluster around the Ashby rail rapid transit station.  Let's examine why.

• It is about halfway between the two big job centers in the area: UC Berkeley and Downtown Oakland.
• It is 20 minutes on rapid transit to the region's largest job center, downtown San Francisco.
• It is close to freeway on-ramps.
• It is on flat ground and within biking distance of a large number of commercial districts.  Bike access is important since transit service in the evenings and on weekends is limited.
• It is not in itself a downtown district.  This means there are a lot of large houses with lots of bedrooms and yards - the preferred housing type.
• There are already other co-ops in the area.

The one co-op not near a BART station -- an outlier near the bay -- is an artists' warehouse.



Ashby Station: Excellent transit service but low density

The Ashby station neighborhood, which currently consists of mostly single family houses, had long been targeted by planners for transit oriented development.  It has an underground rapid transit station with train frequency of every 7 minutes on weekdays and 20 minutes at night and on weekends.

Ashby station neighborhood.  Station is blue, parking lot is black.


However, the official plan to build 300 units of housing on the station parking lots met significant neighborhood opposition.  Existing residents were concerned about losing the flea market that currently operates on weekends in the parking lot, increased traffic, and future upzoning of the area.  The project was put on hold in 2006. 

Shared housing, however, has achieved something similar with no official intervention.

Can the Ashby model be replicated elsewhere?

Using shared housing to increase neighborhood density offers a solution for low density areas where economic constraints or zoning limits the ability to build new housing units.  The following factors are needed for its success:

Multimodal Job Access

Members of a shared house with long commutes tend to move out.  Therefore a location needs good access to members' existing jobs, and potential future ones.  Transit is just one part of the equation - those working 9 to 5 hours downtown.  For the other members, good car and bike access to nearby commercial areas is needed for those working retail and service jobs on evenings and weekends. 

Catalyst community and clustering effect

The Ashby area community began with just a couple of houses.  However, the community grew rapidly as the original residents' friends also wanted to live nearby.  UC Berkeley provided a feeder system with many members having already familiarized themselves with shared housing through living in the Berkeley student co-ops.  In places far from colleges, immigrant neighborhoods can also benefit from the feeder effect.

One of the side benefits of shared housing is a group of residents will have a lot of purchasing power with their combined incomes, and will seek out the area that meets their needs best.  Until most people live in shared housing, this means that a region will likely have only a small number of preferred neighborhoods.

Why not MacArthur?  Station placement matters.

The next station down the line, MacArthur, is even better positioned in terms of access to transit and jobs.  However, this station is located in a freeway median.  As a result, a lot of the land walkable to the station is either paved over, or so close to a freeway that it is an unpleasant place to live.  As a result, even though this area has been zoned for multistory apartments, few have been built.

Creating community: the station as social hub

Shared housing in the Ashby area occurred without any official planning - an existing community (recent UC Berkeley graduates) simply moved in.  Where no existing community exists, social hubs such as coffee shops or community centers, located inside or directly next to the station, can aid the formation of the close relationships that shared housing requires.

basics: conceptual triangles

Sometimes, we have to think in triangles. 

In the transit world, for example, we know that ridership arises from a relationship between urban form (including density and walkability) and the quantity of service provided.  For example, if we focus on local-stop transit, the triangle looks like this:

You can think of development and service as inputs, and ridership as the output, but you can rotate the triangle any way you like.  For example, if you want to justify an increase in service, you want both some existing ridership success and also evidence, from the development density, that there's more potential.  In other worse you want to say "Ridership + Development --> Service"  On the other hand, if you want dense development as an output, you want a transit line that feels permanent.  That's going to be one that already has high service, achieving high ridership. "Service + Ridership --> Development."  (In each case, the right-arrow means "supports" or "helps to generate" or "is one of the crucial necessary conditions for")

I am reminded of this triangle because transit discussions are full of assertions about the relationship of two points on this triangle.  Some of these are quite famous.  Pushkarev and Zupan, for example, are dutifully cited in every article that deals with this topic.  WIth a publication date of 1977, they are one of the oldest must-cite authorities in transit research; it's as though they've passed over into the realm of scripture.   Their conclusion?  "A net residential density of 15 units/acre supports a service level of 10 minutes or better over a 20-hour span."  This is a stated relationship between two of the three points on this triangle, service and development, without reference to the third, ridership.  So hidden inside the verb "supports" is an assumption about what level of ridership is acceptable.

There is, to my knowledge, no agreed standard about what level of ridership is acceptable, nor should there be.  Every city or agency decides that for themselves.  That means that logically there can be no answer to the question "is development pattern and density x sufficient to suppoert service level y?" 

In the British-influenced world, of course, there's often a hidden implication that the ridership standard should be "profitability."  Certainly, if that's your goal, then you've fixed that variable and can now look more coherently at the relationship between the other two; in other words, it then becomes coherent to ask "what service level should be offered in response to each development type, so as to hit a goal of profitability."  (Yes, fare policy plays a part, and that's still another variable, but you can also choose to hold that constant.)

But of course, most of us don't think transit should aim for profitability.  A profitability paradigm assumes that government has no public interest in the success or failure of transit, just as it has no interest in the success or failure of Coke Zero.  And obviously that's wrong.

So when I hear people either quote or update Pushkarev and Zupan, saying that density x supports what level of service y, I hear pure incoherence.  The statement says nothing that's useful to me.  What am I missing?

UPDATE:  Jeffrey Zupan himself wants to assure us that his work was based on a clear ridership assumption, expressed in the form of targets for cost/passenger and cost per passenger mile.  Unfortunately, most people who quote his work forget that part.  In doing that, they're implying that the ridership target that Pusharev and Zupan chose is the right one for their own community, even though they don't know what it is!

can we make density make sense?

When commentators argue about the importance of density to public transit, it's common to hear a range of density figures thrown around.  These figures are almost always averages over a large area, and as I argued on my recent post on the perils of average density, averages are simply useless for describing the kind of density that determines public transit outcomes.  

As Paul Mees's new book shows, you can easily run numbers that show that metro Los Angeles is denser than metro New York. If you want to sow confusion, you'll use those numbers to imply that since Los Angeles has far lower transit ridership than New York, density must not matter to transit after all.  In fact, density matters, but average density doesn't.  What matters to public transit is the density right around its stations and stops, and this can be quite unrelated to the average density of a whole urban area.

The post is still worth a look for its excellent comment thread.  Several commenters proposed alternative ways of talking about urban density that would better describe the way density matters to transit.  They all seem to have the same theme:

The conventional idea of density is about comparing units of area to each other: this patch of land is denser than that.  Instead, compare the densities of units of population.  

There seem to be two ways of asking a population-centered density question:

1. What percentage of the population lives at densities above x?
2. What's the average density at which people in this city live?

On the blog We Alone on Earth, Fedor Manin took a shot at the question "what percentage of the population lives at each density?"  Here are his graphs:

The second graph is a distribution of the population by density, showing what percentage of the population is in each density range.  Unlike the average densities of metro Los Angeles and metro New York, which I discussed in my last post, these graphs get closer to what matters for transit.  In New York, (the pale blue line) you have a lot of people living at very high density -- inevitably right next to a subway station -- and these are what drive the city's high transit needs and performance.  In Los Angeles (the dark blue line that peaks in the center of the graph) you have an overwhelming predominance of a moderate density figure.  That moderate density corresponds to development forms ranging from the townhouse to the four-story apartment building, but this form requires long travel distances and more spread-out access needs than New York does, so it yields much lower transit performance.

Notice how this graph corrects common stereotypes of Los Angeles, stereotypes that you might find reinforced by Google Earth.  A lot of the area of metro Los Angeles is large detached homes with swimming pools, a very low density.  So if you're thinking in terms of area -- as anyone looking at a map must implicitly do -- that development pattern seems to dominate the city. 

Yes, but although those stereotypical homes cover a lot of area, they don't represent all that many people!  That, after all, is what low density means!  So if you look what percentage of the population of Los Angeles that lives in these low-denstity homes, it's not much.

Now here's the point that's easiest to miss:   All that area covered by relatively few people is not really a problem for transit, so long as we don't give it more service than its relatively small population (and low ridership potential) deserves.  Transit outcomes don't really depend on how much of this low density stuff there is in a city.  They depend on how much high density stuff there is, and the average density of the whole city will tell you nothing about that.

Fedor Manin's distributions of population by density, charted above, still don't get us exactly to a citywide assessment of "density as it matters to transit," but they are much, much closer than any citywide average density.  On the other hand, a distribution can't be reduced to a single quotable number, and in many conversations, if you don't have a single quotable number, you don't get a word in.

So the answer, as several commenters have suggested, is probably to talk about the average density per person rather than per unit of area.  It may seem that density is intrinsically a "per unit of area" concept, but you can certainly ask:  "At what residential density does each citizen live?"  Here, residential density is ideally the density of a single parcel: one house or apartment building.  What percentage of the population lives on detached homes on quarter-acre blocks?  What percentage lives in buildings taller than five stories?

Then, you can ask "What's the average residential density at which people in this city live?"  Again, this is an average over the population -- not, like conventional density, an average over the area. 

This is probably still too much averaging, and likely to mislead, but at least it takes out a lot of the noise that contaminates citywide averages, and focuses on fine-grained parcel-level patterns that are more likely to reflect proximity to transit.  Readers are encouraged to play with this idea!  

the perils of average density

In his 2010 book Transport for Suburbia, Paul Mees notices a fallacy that seems to be shared by sustainable transport advocates and car advocates.  Both sides of this great debate agree that effective transit requires high density. 

Sustainability advocates want higher urban densities for a range of reasons, but viability of public transit is certainly one of them.  Meanwhile, advocates of car-dominance want to argue that existing low densities are a fact of life; since transit needs high density, they say, there’s just no point in investing in transit for those areas, so it’s best to go on planning for the dominance of cars. 

Mees calls on his fellow transit advocates to let go of the idea that good transit requires high densities:

The central argument of this book is that density is not destiny.  Transport policy itself has a bigger impact on transport patterns than urban planners have realized, and suburbs don’t have to be totally reliant on the car.  Planners who insist that car dominance can only be addressed by impossibly large increases in density may actually be entrenching the problem they are trying to solve.

Transit does depend on density, but there are three problems with saying that “transit requires high density.”   

• First, it offers no hope to places that are already built at low densities and unlikely to change. 
• Second, it gives public transit in low-density areas an excuse for descending into a cycle of underinvestment, official neglect, and lazy operations, because after all, nobody expects public transit in low density areas to be any good anyway.  
• Third, it invites sweeping claims about transit’s viability based on the overall average density of a city, claims that, as we’ll see, make no sense at all.

Mees is right that “density is not destiny.”  Cities of any density can make better or worse transit choices and achieve significantly different outcomes as a result.  But density is still an overwhelming force in determining the possibilities and outcomes of transit, and we can’t begin to make good transit decisions until we understand it.

Density is hard to talk about for two reasons:  It’s very emotional and it’s hard to define and measure. 

The emotions are inevitable: Whenever we talk about urban form, people hear us making judgments about their homes.  I can stand in front of a group of citizens and talk about how a certain kind of development pattern implies certain consequences for transit, and thus for sustainability, and thus for civilization.  As we talk, it may appear that we’re having a thoughtful and educational discussion about good and bad design.  But some people in the audience have chosen to make their homes in the very development pattern that I’m describing, and to those people, I’m saying that their home is good or bad. 

Once you hear that, you’re likely to have a strong emotional reaction that makes you deaf to rational argument.  On some level, consciously or unconsciously, you’re going to feel as though I’d walked into your own livingroom and told you that your decor is not just ugly but a threat to civilization.   We all feel defensive about home.

But emotions often hide inside things that look like facts, and density "facts" are a great example.  In transit arguments, people say things like “the net density of Toronto is ###/hectare,” because this sounds like a fact and therefore conveys some authority to their argument.  In reality, though, there are several possible meanings of “density” in that sentence, as well as several possible definitions of “net” and ”Toronto.”   Briefly:

• “Density” in an urban planning context is always some kind of quantified human presence divided by some kind of land area, but it can be residential density (residents or homes per acre) or it can be total development density -- including homes, businesses, schools etc – or it can be an economic density such as the number of jobs in an area. 
• “Net,” as opposed to “gross,” means that the human presence is being divided by a smaller unit of area instead of a larger one.  In calculating the area, for example, you might take out undevelopable land, and bodies of water, and the land taken up by streets and highways -- or you might not.  There are arguments for or against excluding each of these things from “net density,” and an opinion about each of them is hiding inside the word “net.”
• “Toronto,” of course, can be the City of Toronto, or the Toronto Transit Commission area, or the whole urban mass of greater Toronto.  These obviously have utterly different average densities.

So the statement “the net density of Toronto is ###/hectare” is really as subjective as the statement “I think that cities should have more open space, narrower streets, and should have single governments covering the entire urban area.”  Because each of those opinions can affect how you choose to define “net,” “density,” and “Toronto,” which in turn determines the number that you declare, with cold factual authority, to be the net density of Toronto. 

At the core of Mees’s book, for example, is a table[1] that compares the gross residential density of a bunch of urban areas with the transit performance in each area.  The point of the table is to show that there’s really no relationship between the two.  Before we look at some fun facts from this table, remember that definitions are everything.  So in this case:

• “Metro Area” (my term for what Mees’s table calls “City”) is the entire urbanized area around a city, regardless of government boundaries.[2]  Think of it as the continuous patch of lights that you can see from an airplane at night.
• “Density (pop/ha)” is residential density – the entire population divided by the entire area. 
• “Transit work mode share” is the percentage of trips to work that go by transit, as opposed to car, walk, bike, etc.  Trips to work are not the only ones that matter, but they are the only ones that are measured in a consistent way.

So with those cautions, here’s a taste of Mees’s scandalous numbers:

Metro Area	Density (pop/ha)	Transit mode share for work trips.
Los Angeles	27.3	4.7
New York	20.5	24.8
Las Vegas	17.7	4.1
Vancouver	17.0	16.5

The density figures are calculated to shock.  Los Angeles is denser than New York?  Yes, that’s what happens when you average over a huge contiguous metro area.  Your own impression of “New York” is probably formed by the massed and soaring towers of Manhattan, but New York is a dense city surrounded by a lot of very low-density suburbs that are all included in this concept of “metro New York.”  Likewise, your stereotype of Los Angeles may be a ranch-style house with a big pool on a cul-de-sac, which implies a very low density, but in fact much of greater Los Angeles is four-story apartment buildings lined up for mile after mile.  Obviously, our impression of the density of a city is based on the part of it we choose to remember, or that we tend to see on television.  The average density of the whole urban area may be entirely different.

To me, Mees’s table proves that average density over a whole urban area is the wrong kind of density for understanding transit.   The impression you probably have of the densities of these cities is actually closer to the kind of density that matters. 

Transit reacts mainly with the density right around its stations.  It is in the nature of transit to serve an area very unevenly, providing a concentrated value around its stops and stations and less value elsewhere.  So what matters for transit is the density right where the transit is, not the aggregate density of the whole urban area. 

Of course, what matters even more precisely is how much stuff is within walking distance of a station, so it's not just density (the amount of stuff in a fixed radius) but the completeness of the pedestrian network.  A poorly connected pedestrian network can ensure that much of the stuff that's within a 400m radius is not in a 400m walk. 

Consider Las Vegas, which Mees finds to be denser than highrise Vancouver.  The Las Vegas economy is based on hospitality and entertainment, labor-intensive industries with low average wages.  So the city needs a lot of housing suited for lower incomes.  The Las Vegas way is to build utterly car-dependent apartment buildings on a vast scale, achieving all of the disadvantages of density with none of its benefits.  Consider:

For scale, the width of this entire image is about 0.55 miles, or 0.9 km.  The arterials along the south and west edges of the image are the only streets straight and fast enough for efficient transit.  Note how these large apartment developments are soundwalled from most direct access to these arterials, and how the accesses they have tend to come out at points where there's no controlled intersection, so the arterial is dangerous to cross.  (Two-way transit trips always require stops on both sides of the street, so if you can't cross the street, you don't really have access.)

So yes, Las Vegas has massive quantities of apartment buildings, which yield a high average density.  But at two levels of scale, these are deployed in patterns that make effective transit difficult.  On a macro level, Las Vegas is mostly midrise apartments spread over a large area, requiring transit to cover more distance to serve them; this is the most obvious explanation for Las Vegas's low transit performance compared to highrise Vancouver.  But the micro explanation is important too.  In the details of street pattern and pedestrian circulation, typical Las Vegas urban fabric is designed for motorists and hostile to pedestrians.  Average urban density says nothing about either of these factors, even though they are what really determine the transit experience in each city.

It's inevitable that in comparisons between cities, we're going to hear average density figures.  These figures are better than nothing when it comes to making global comparisons.  The famous databases created by Jeffrey Kenworthy and Peter Newman, for example, are useful in showing how a range of sustainability outcomes vary with density. 

But averages are dangerous, especially in transit.  Averaging sounds like a credible way to draw a simple fact out of a huge and diverse reality, but often, averages are just not the fact that matters.  In a recent post on the perils of average success, I pointed out that no transit customer really cares about average frequency, or average lateness, or average crowding.  By the same token, transit advocates need to be suspicious of average density.  What matters is the density around the transit system.  How can we best measure that?

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[1]   Fig 4.1 p 60

[2]   In the US, “an ‘urbanized area’ is defined for each metropolitan region, made up by combining adjacent ‘census blocks’ (the smallest units for which data are collected) with more than 1000 residents per square mile, or 386 per square kilometre. …  Canada defines “urban area” on an almost identical basis to the US, using a density threshold of 400 [residents] per square kilometre.”  The data is from 2006 census in Canada, 2000 in the US.