Do Cities Densify or Disperse as They Grow?

In a recent post, Chris Bradford has built on some of the findings I made on weighted density last month to show how housing affordability has relatively little relation to density.  In the graph below, I have a look at a related subject: the relation of total population to density, to determine whether cities tend to grow denser as they grow larger.  I use urbanized, rather than weighted density, since the purpose here is to determine the relative change in extent of the built-up area at various population levels, and plot it against MSAs ranked in order of population (not population itself). A trendline with period 20 is overlaid on the scatter plot.

As predicted by the correlation data from an earlier post, urbanized density is here shown to be significantly related to total population, yet the scatterplot teases out some intriguing nuances in the data.  For instance, urbanized density shows little change for MSAs up to a population of around 200,000 (around point 150 on the X axis).  A modest upward trend is visible for cities between 200,000 and 700,000, after which the line slopes sharply upward. How to explain these trends?

An economic model using a simplistic urban land value gradient (illustrated as the New Urbanist transect) would find the results entirely predictable: as a city grows in extent, the time value of a central location becomes increasingly large relative to the price value of a peripheral location, such that we'd expect to see a slowing in the rate of growth of the urbanized area over time as either 1) new residents locate in existing, centrally-located neighbourhoods, 2) new greenfield developments are built at higher densities or both.

Using averages of urbanized area density for each population level, it is possible to visualize this process in action.  At left, I show the travel time in minutes from fringe to center (assuming for simplicity's sake a perfectly circular and monocentric city with no topographical impediments, and using the mean US commuting speed of 32 mph) holding density constant at 2,000 people per square mile, which represents the average density for MSAs of below 400,000 inhabitants.* At right are the same figures using the densities that are actually observed for each of the population ranges.

For smaller cities, even significant increases in population do not cause travel distance to the center to increase to unpleasant levels (bearing in mind that the typical person wants to devote no more than one hour per day to commuting), so densities do not increase – in fact, for cities less than 150,000, growth appears to track with a slight decrease.  Significant increases in urbanized density kick in only when travel distance from fringe to center approaches 20 minutes, and rapidly increase thereafter in an apparent attempt to keep maximum one-way travel time close to 30 minutes.  Were larger cities comparable in density to smaller ones, one-way travel times from the urban edge could approach one hour for cities of around 5,000,000.

At this point, I might expect Wendell Cox to interject with his contrary finding that "the general tendency is for cities to become more dispersed (less dense) as they grow."  Cox's conclusion was drawn from a statistical study of urbanized area density for cities from each of the 1950 to 2010 censuses, in which he found that most have indeed become less dense over time even as their populations increased dramatically during this 60-year period.  How can this be reconciled with the figures above?

In a nutshell, because the 1950-2010 time period covers a transportation, zoning and family planning revolution that completely altered commuting patterns and household composition.  In 1950, the interstate highway system did not yet exist, and relatively little new housing had been built following the emergence of zoning in the late 1920s due to the Depression and the lean war years.  The combined effects of these two developments – speedy access to city hinterlands combined with rules that prevented intensification of and discouraged investment in existing neighborhoods – no doubt contributed to a massive decentralization that overrode the natural tendency for urbanized density to increase with growth.  Additionally, household size contracted as the birth rate declined, which would tend to cause a steady decline in population density even where the concentration of housing units remained constant.

In fact, examining the fastest growing cities during the period 1980-2010, after the completion of the bulk of the interstate highway system, there is clear evidence of a swing back toward higher urban densities, particularly in those places that had little pre-automobile urbanism to de-densify either through abandonment or gentrification. Las Vegas, for instance, which had a population of 25 at the dawn of the auto era, is almost twice as dense as it was in the 1980 census. In Phoenix, lot sizes for new single family detached homes declined steadily after peaking in the late 1970s, leading to an odd situation in which neighborhoods on the urban fringe are often denser than much more centrally located ones. Notable exceptions include Southeastern boomtowns like Atlanta and Charlotte which, however, saw their density declines slow or cease after 1980.**

These numbers help provide a partial explanation for the data in an earlier post, Commutes, Tradeoffs and the Limits of Urban Growth, where I noticed that mean commuting times did not increase in step with population growth, and provides further reassuring evidence that even automobile-based urban expansion (i.e. suburbanization) contains a natural braking mechanism that will eventually slow the rate at which new land is consumed for development.

*Of course the average travel distance to the center will be less for the MSA as a whole, and most new residents will not be commuting to the center at all, typically reducing their commuting times.  The model is a deliberate oversimplification intended to illustrate basic trends.

**Interestingly, both Atlanta and Charlotte pursued major mass transit projects around this time period (MARTA's heavy rail system and the LYNX light rail, respectively) while much denser Las Vegas did not.  One could speculate that car commutes in Las Vegas were much shorter due to its density-driven compact urban area, leading to less political pressure for alternative forms of transportation to serve suburban commuters (despite being considerably larger than Charlotte population-wise, Las Vegas currently occupies only 56% of its land area).  Perhaps coincidentally, Atlanta and Charlotte, of very similar densities, approved tax increases to fund rail service at around the same respective point in their population development – Atlanta at 1.8 million, and Charlotte at 1.5 million.

Yet Another Height Act Article

Washington's Height of Buildings Act has been a subject of debate for years now, but lately, with the Act under Congressional scrutiny, the discussion has taken on more than academic significance.  I'm wary of touching the subject at all, since it seems to have become more ideologically polarized than almost any other planning-related topic, generating ample heat but less light (one recent article refers to economists responding to its mere mention with "paroxysms of outrage").  Points of view ostensibly grounded in economics and statistics are supported with flip references to generalized principles, comparative urban study is given short shrift where it is not ignored entirely, and each side accuses the other of using economics as thin cover for aesthetic preferences, whether it be the free marketeer's skyscraper envy or the traditionalist's veneration of the mid-rise city.

A thoughtful article by David Alpert in Greater Greater Washington has triggered a vigorous debate, with Kaid Benfield mustering all available arguments in the Act's defense, and Alex Block and George Mason law professor David Schleicher on hand with rebuttals. Graciously citing an earlier post of mine, transportation planner Dan Malouff has issued a rebuttal to the rebuttals, taking a much-needed big picture look both chronologically and geographically, and touting the benefits of the Act for good urbanism. 

Without venturing into the aesthetic and form-based arguments, which have been covered exhaustively elsewhere, the hard numbers and economic arguments that have been submitted in opposition to the Act are relatively scarce.  I'll attempt to address a couple of them.

This recurrent complaint of high office rents raised by the free market side has made its most recent appearance in Schleicher's article, where he mentions that "[o]ffice space in downtown D.C. is more expensive than in New York's financial district, and 850 square-foot apartments in Anacostia, one of the cheapest areas in the city, now rent for $1300 a month" (yet citing to an article which states that in 2010, DC office rents were higher – barely – than those of New York City, not the financial district).  In response I would note:

• The number appears to be cherry-picked. The same article cited by Schleicher indicates that rents had been much higher in New York during the preceding five years, and a more recent study of Class A office space in the DC and NY metropolitan markets shows DC rents at only 78% of the level of New York.  The 2010 market figure, taken near the bottom of the real estate market, does not appear to be representative of long-term averages, nor is it entirely clear what the geographic unit for comparison was (comparing DC proper to NYC would tend to inflate DC rents relative to NY rents, for instance, as DC is a much smaller fraction of its greater metro area).
• Taking into account median incomes, both DC's office rents and residential rents seem cheap compared to NYC, with office rents the cheaper of the two. In recent years, the DC metro area has had the highest median household and individual income in the country, considerably higher than New York.  Based on this figure, we should expect to see very high housing costs (and probably also office rents) in the DC region, as income and housing costs are highly correlated throughout the United States, and DC moreover has other unique office demand factors related to the presence of the federal government. Washington does in fact have the second-highest MSA housing costs east of California, yet these are still cheaper than New York's, with the result that DC is in housing-to-income terms far more "affordable" than New York.  And yet office rents appear to be cheaper still: DC's housing cost averages 89% of that in New York, while its office rents, averaged over the past several years, seem to be in the range of 70-80% of New York's.  This is the opposite of what one would expect based on the views of the Height Act opponents, whose argument supposes that the Act primarily impedes construction of high-rise office space in the central business district.

Apart from a citation to a number from an Ed Glaeser study, which was partly debunked as regards its application to the Height Act in another Atlantic Cities article, there are no other figures presented in Schleicher's article.  Schleicher does cite, as is common in this debate, to general supply and demand principles, but Malouff counters him, noting that:

"There is currently around 100 million square feet of office space in downtown DC, which makes it the 3rd largest downtown in America after New York and Chicago. Despite no skyscrapers, downtown DC currently has a greater supply of office space than downtown San Francisco, Boston, Philadelphia, or Los Angeles."

So, broadly speaking, we do have a result that complies with supply and demand principles: DC has a larger supply of office space than comparable cities, and has office rents relatively lower than we would expect based on median incomes and other comparative measures.

I would add that, according to a 2006 Demographia survey, Washington's CBD is consistently found to be larger and denser than those of its peers.  For instance, DC's CBD is 1.5 times larger and has 1.7 times the employment density of Houston's. Similarly, it is both 1.3 times larger and denser than Philadelphia's CBD.  In both cases, it achieves this despite having less buildable area to work with, due to DC's exceptionally wide streets and generous allotment of parks.  It is also much more centralized, containing 18.7% percent of metro employment compared to Houston's 8.9%.  Given all this, can we at least allow for the possibility that the Height Act has actually stimulated, rather than impeded, growth and intensification of the downtown business district?  This would of course have implications for the usefulness of height limits to other cities and in other contexts, but it's a topic that deserves greater study.

Whatever the true answer might be in this case, more empirical evidence would be helpful in figuring out  answers to the economic questions at issue here.  Throwing around terms like supply and demand, however, without bothering to investigate what the “supply” is the in the first place, does not greatly contribute to the discussion.  One hopes that the forthcoming Congressional study is up to that task, but with the little evidence-gathering that has been done so far seeming to have been in the service of pre-determined conclusions, I am keeping my expectations low.

Exploring Weighted Density

Chris Bradford has recently run an excellent series of posts featuring the Census Bureau's newly-released data covering population-weighted density.  Chris has been an advocate of this density measure (also referred to as "perceived" density) for much longer, though, and has promoted it as a more useful alternative to both standard density and the somewhat more helpful urbanized area density.

At The Atlantic Cities, Richard Florida picked up on the story, noting its significance of these new figures in exploring the relationship between density and productivity.  The topic has been addressed at least once before: a 2010 report from the Federal Reserve Bank of New York, updated last year, used weighted population density to find that:

• In general, productivity increases by 2 to 4 percent as weighted density doubles.
• Productivity increases are correlated with human capital (e.g. skills and education), such that cities with a human capital one standard deviation below the mean have no productivity gains from increased density, while those with high human capital have twice the average gain.
• The benefits of density are especially pronounced for certain industries, including professional services, arts, entertainment, information and finance.

Along the same lines, I've drawn up a correlation chart showing the population-weighted density for all metropolitan statistical areas as compared to several other factors, including total metro area population, population change during 2000-2010 both in net and as a percentage, urbanized area density, median home values, median personal income (a stand-in for productivity in many studies), and finally income-to-home value ratio (an indicator of relative housing affordability).  Each factor has been compared against every other (raw data is available here).

A few of the things that jumped out at me from the chart:

• Income is more strongly correlated with weighted density than total population, although not dramatically so. However, median home values were even more strongly correlated with weighted density.  The result is that, for cities of equivalent size, the city with the higher weighted density will generally be less affordable in relative terms, even if incomes are higher (for instance, Sacramento is almost twice as dense as similarly-sized and lower-income Kansas City, but is only two-thirds as affordable).
• Although high weighted-density metros have generally higher incomes than low-density cities, they grew more slowly than these cities, perhaps indicating the push and pull forces of housing affordability.
• Nonetheless, relative housing value was negatively correlated with population growth, although not strongly.  This suggests a tension between low housing values being a product of low demand, and the attraction of low housing costs in otherwise prosperous cities that have presumably kept prices low through adequate supply.  Looking at only large cities bears this out (see below).

Here is the same chart showing only MSAs with more than one million inhabitants as of the 2010 Census:

The sign for affordability relative to population gain has flipped, and more affordable markets are here associated with higher population growth. 

Another factor I would have liked to include, had it been available for all MSAs, would have been median transportation cost, since to some extent that would offset the poor affordability of certain high-density cities.

I am skeptical, though, that these figures would make much difference for most cities.  Although recent studies have pointed to the transit savings of living in high density areas well-served by mass transit, most American metro areas remain overwhelmingly car dependent. Moreover, while high housing costs cannot be easily avoided, households have more direct control over transportation spending even in low-density cities. 

Beyond these points, I'll leave the numbers out there to speak for themselves.

Places That Aren't Car-free, But Should Be

The list of truly car-free urban settings in the world is surprisingly small, and for the United States, virtually non-existent outside of a few pedestrianized main streets. Tracy Gayton is leading a visionary effort to construct a completely car-free village in rural Maine according to traditional urbanist principles, in what would be the first such place of its kind in the United States, but I thought it might be worth pausing to consider what existing places would be best suited for a car-free retrofit.

In particular, I want to focus on those places in which the introduction or continuing presence of the automobile is seemingly irrational, yet the car appears in great numbers nonetheless. The incentives and motivations are simple: the first person to introduce a car to a given location obtains all the benefits of auto transport and suffers virtually none of the costs, which are instead imposed on others. Once a few cars are whizzing around, however, the collective advantage of a car-free city is lost, and a person who continues to refrain from purchasing a car only denies himself the benefits, as the costs have already been imposed on him by others.

Nathan Lewis talked about a similar phenomenon in the case of Juneau, the car-inaccessible Alaskan capital which has been built almost entirely around the needs of the automobile.  In the cases below, the towns and cities themselves are perfectly well-suited to pedestrian and/or bicycle transit, or nearly so, but have come to rely on motorized power to move human beings around anyways.  I'm sure readers can think of many others, but this is a start.

The City of Malé

Malé; the red line shows a 5-minute walk.

Malé, the capital city of the Maldives, accommodates over 100,000 inhabitants on a tiny island of just two square miles, roughly the same size as Barcelona's Gothic Quarter. Almost entirely covered with row houses and mid-rise apartment buildings along narrow streets, the island should be a pedestrian's paradise. It is ideal for bicycling, too, since the climate is balmy year-round and the island is flat.  The longest possible walk from one point to another on the island is just over 20 minutes, and the walk from the ferry to the center of town is less than 10. Commutes of longer than 5 minutes are impossible unless one deliberately forgoes a bicycle.  If that weren't enough, the island has a bus system as well.

In spite of this geography, photographic evidence shows the streets of the city utterly clogged with motorbikes, cars and trucks. One photo even shows a large commercial building with two floors devoted to a motorbike dealership. A recent article depicts the city government struggling to impose a driving ban on the single day of the Muslim holiday of Eid-al-Fitr, suggesting at least indirectly an awareness of the effects of mass motoring on this island.

The situation of the nearby island of Hulhumalé, built up through expensive and laborious land reclamation, is even less forgivable since it has been deliberately planned.  The home page of the island's development corporation shows that the primary development strategy  is the construction of numerous wide and unattractive asphalt streets, which are prominently featured in photos on the website. This is confirmed by looking at an aerial view. A purpose of these streets, according to the developers, is to "reduce congestion."  The types of transportation modes that might cause this "congestion" are not mentioned, although by the dimensions of the streets the planners are clearly anticipating cars, and lots of them.  This on a flat tropical island which takes three minutes to bike across!  There is also a rigid system of Euclidean zoning in the works, with the purely residential areas located on the opposite side of the island from the shopping areas, with large green spaces, an academy and governmental buildings serving as a buffer between them. Perhaps this was to encourage residents to purchase cars and motorbikes lest the streets go unused.

Nantucket

This choice is only natural, since the island of Nantucket was one of the last towns of any significance in the United States to retain a ban on cars, only relenting in 1918.  Although the island is far larger than Malé, population is concentrated in a single town approximately a mile by a mile and a half.  (A small settlement on the eastern end of the island was formerly served by a narrow gauge railway, and today has seasonal bus service.)  Similar to Malé, the terrain is largely flat and the weather is mild for much of the year.  Like a handful of other fishing and whaling villages of the Massachusetts shoreline, it lacks most of the telltale characteristics of the 19th century hypertrophic American city. Streets are narrow, a grid is absent, and setbacks are frequently small or non-existent. In form if not architecture it very much resembles Japanese urbanism, and is particularly well-suited for a pedestrian or bicycling culture. Most residents already get about by walking or on bike and the island has an extensive network of bike paths – why not take the next logical step and reclaim the island entirely for human beings?  (At right, Nantucket street vs. Tokyo street).

Santa Catalina Island

Although California's Santa Catalina Island does ban large vehicles, smaller cars and golf carts are permitted, and flood the streets of the tiny town of Avalon. Why this is necessary is not obvious.  From one end of the town to another is a five-minute walk and, according to Google maps, a two-minute bike ride.  The island also has a near-perfect climate with mild weather year-round and little rain in most months.  There are no other large settlements on the island. Where are all these people driving to?  (One could also ask: since the town was platted before the invention of the car, why were the streets designed to be 28 feet wide?)

Manhattan

This one is on a slightly larger scale. The idea of banning most, if not all, private cars from Manhattan is not new.  In 1961, the same year Jane Jacobs published The Death and Life of Great American Cities, writer Paul Goodman and his brother Percival authored an essay proposing to pedestrianize the majority of Manhattan's streets and avenues.  The crux of the Goodmans' argument was that private cars, in the manner in which they are currently used in Manhattan, "are simply not worth the nuisance they cause."  Ahead of their time, like Jacobs, the Goodmans also suggested a congestion charge to achieve some of the same benefits.

Whether one favors a bold approach like the Goodmans advocated, or a Jacobsian strategy of attrition of automobiles, the fact remains that New York has failed during the half century since in providing much new permanently car-free space.  Only the recent and partial pedestrianization of Times Square stands out.  Manhattan and New York as a whole continue to lack a single pedestrian shopping thoroughfare, such as Buenos Aires' Calle Florida, Copenhagen's Stroget, or Shanghai's Nanjing Road.  As the Auto-Free New York site has recently pointed out, the city failed to even maintain Roosevelt Island, New York's island within an island, as a carfree sanctuary, although it had been planned as one. In spite of these disappointments, it is difficult to think of a better and more appropriate setting for the creation of new pedestrian areas.

Bonus: A Car-free Town That Behaves Like It's Not

Mackinac Island, along with New York's Fire Island, is one of the best-known of America's genuinely car-free summer vacation spots.  Truly living up to its billing, the island does not even have motorized vehicles for mass transit, and instead relies on horse-drawn wagons and bicycles. 

Flickr/andrew.petro

The design of parts of the town, though, is unmistakably 19th century American hypertrophic, with a wide main street that looks much like any other from its era.    Despite the lack of cars, there are raised sidewalks on each side of the street, and a blacktop-paved carriageway.  The street is still designed for the benefit of wheeled vehicles, not people on foot.  Subconsciously obeying these spatial cues, people walking about the town (see photo at right) appear to mostly remain on the sidewalks, while the street itself accommodates wheeled – although not motorized – traffic. So, although the town is in fact car-free, this has meant less than might be expected in terms of how people make use of public urban space.  

The example of Mackinac Island shows that the absence of cars, alone, is not necessarily sufficient to create a pedestrian-friendly urban space.  Design matters too.

Friday Read: Residential Streets and America's Hidden Parking Policy

With discussions about on-street parking tending to focus on high-demand metered spaces in retail districts, the fact can get lost that the overwhelming majority of on-street parking in a given city serves exclusively residential areas.  We tend to take these spaces for granted: in a low-density area, they typically have very low occupancy rates, making them almost invisible.

These spaces are the subject of a new study, Amenity or Necessity? Street Standards as Parking Policy, which examines the "hidden parking policy" implicit in the standards for street width established by many cities.  The study calculates that these standards have produced somewhere between 740 million and 1.5 billion parking spaces along American residential streets – enough to host all of the passenger vehicles in the world – the vast majority of which are unmarked, unmetered and indeed unused. The authors estimate that the cost of construction of these spaces is in the trillions of dollars, with an annual maintenance cost in the tens of billions. These costs, the authors note, amount to approximately $1,000 annually per home.

Murfreesboro, TN street, 36 ft. wide, built circa 2005.

This parking provision is the result of minimum width requirements which, by their dimensions, automatically prescribe parking lanes (a 36-foot minimum, for instance, effectively provides two parking lanes).  Noting that a requirement for wide streets is equivalent to a requirement for on-street parking in most cases, the authors interviewed a series of city transportation planners to ask a) the basis for mandating street parking and b) the basis for minimum street widths.

Interestingly, although the resulting streets often look the same under both objectives, planners claimed that the purpose of providing on-street parking is to serve as an amenity for residents and visitors, whereas minimum street widths are justified for safety reasons.  As the authors note:

"[Most] respondents believed that the purpose of mandating parking was to provide extra parking, which was accomplished through the minimum street width requirement under the guise of traffic safety (rather than parking demand). In other words, street parking is an amenity, but it is provided in the name of necessity. Such 'flip flop' reasoning reflects local decision makers’ ambiguous understanding of the basis for mandating parking in street standards."

Stranger still, the authors found that width requirements for private streets are generally narrower than those for public streets, which calls into question the safety justifications behind mandated public street widths as well as undermining the amenity argument.  In Las Vegas, for instance, public streets are required to be 37 feet wide, while private streets need only be 28 feet.  Of the many traffic planners surveyed, none were apparently able to convincingly explain the discrepancy between the public and private standards in their cities.  In light of the substantial costs associated with street construction and maintenance, however, these differential street mandates effectively serve as a subsidy for private neighborhoods.

Looking at the issue as a whole, the authors further explain:

"The key problem for the parking mandate implicit in street standards is its hidden nature. ... Street parking policy has typically been buried in street width requirements, which are supposedly based on safety concerns rather than parking demand. Many street standards do not even mention parking in descriptions of the minimum width requirement, creating the impression that these requirements solely address street needs and technical issues.

"This "camouflage" makes parking policy invisible to the public and precludes public oversight. In sharp contrast to minimum off-street parking requirements, the street parking mandate has rarely been publicly discussed or debated in the United States. Even New Urbanism supporters do not oppose street parking but allow it on narrow residential streets ... The hidden nature of this parking policy grants it legitimacy because providing streets has been widely acknowledged as a key government function. The issue could become more controversial if this function of providing streets was modified to include "providing parking..."

 The study closes with two recommendations:

• Unmask the hidden parking policy and subject it to public debate.
• Eliminate the double standard between public and private streets, and make parking optional for residential streets.

Although the authors don't delve further into the issue in this study, I think there may be a simpler explanation for the emergence of on-street parking. A study by Michael Southworth and Eran Ben-Joseph suggests that the trend toward requiring wider and wider streets in the first half of the 20th century was in part driven by a conflict between the of the intent of the transportation engineering profession to allow unobstructed, free flowing two-way traffic on all streets and the reality that, once streets were made sufficiently wide for this purpose, drivers would, out of convenience if not necessity, simply park along the curbs, narrowing the street back down again.

With planners' recommendations that travel lanes be 11 feet in residential areas, and the minimum of seven feet occupied by a parked car, this implied roadbed widths of at least 36 feet - precisely what planning commissions and highway engineers had begun advocating for by the 1950s.  In this view, on-street parking was merely a incidental effect of designing for unobstructed movement, which would help explain the confusion current planners have in accounting for the existence of both on-street parking and street width minimums, as well as the identification of speeding as the greatest problem facing residential streets.

This feedback loop of increasing required minimums proceeded in spite of the growing adoption of off-street parking minimums in the 1950s, although these produced little change in form for very low-density suburban areas.  In combination with the higher density suburbs of the more recent past, however, these regulations created something that was the worst of both worlds: very wide streets so riddled with curb cuts that on-street parking was hardly available anyways (see above image). 

This approach was not shared by other countries: Japan, as Paul Barter describes, adopted proof-of-parking requirements for car buyers and helped enforce the policy by continuing to build residential streets so narrow that on-street parking was essentially impossible.  It may not necessarily be the best or most appropriate policy for other cities or countries, but it at least shows a logically consistent approach to the issue.

The question of how street standards and parking supply interact, though, is a key and underexplored issue, and this study may help open a needed discussion on topic.

(h/t Laurence Aurbach for the study).

Commutes, Tradeoffs and the Limits of Urban Growth

I've been looking through the mean commuting times recently released as part of the 2010 ACS estimates for metropolitan statistical areas.  Although a lot has been written about commutes recently, what I found most striking about the data was the similarity between mean commuting times among large metro areas, regardless of their population.  Below is a list of all American MSAs with more than two million residents for which mean commuting times were available through the ACS:

Although population is strongly correlated with commuting time, particularly when all metro areas over population one million are included in the sample, the differences are surprisingly small.  Orlando, with an urbanized area of only 600 square miles, has a mean commute just twelve seconds shorter than Dallas, which covers 1,780 square miles, although both cities have comparable densities, employment centralization and highway miles per capita.

The conventional explanation for this phenomenon, as one 1997 study puts it, is that "individuals and firms mutually co-locate in response to congestion costs, and thus reshape those costs."  Implicit in this "tradeoff" theory is that, for most people, commutes beyond a certain length of time are undesirable despite any other advantages that might be gained from the location (e.g. housing cost, school quality, taxation level, crime), or else we should see commute lengths increase at a faster rate relative to population.

The data suggests that maximum mean travel time is somewhere slightly over 30 minutes, as indicated by the outlier example of New York, where most residents of the metro area apparently prefer to pay very high prices for housing rather than relocate to far cheaper, but more distant locales (such as Dutchess County or the northern Philadelphia suburbs). The 30-minute figure has been noticed before by transportation planners.

What do these constraints suggest for the urban form of America's cities, if anything?  The map below shows the commute from a new residential development in Katy, Texas, to Houston's central business district, which is measured by Google maps at 44 minutes in optimal traffic conditions (Uptown Houston is only a slightly closer 39 minutes).  At distances like this, with a diminishing number of potential purchasers willing to undertake such long commutes to major job centers, faster transit virtually out of the question, and any new edge city employment clusters unlikely to contribute a significant proportion of metro area jobs, one wonders how much longer outward urban expansion can continue at its current rate.

If the theory that commuting time preferences are more or less universal is true, we should by now expect to see a considerable refocusing of residential development in and around Houston's employment centers (bearing in mind that less than 1 in 5 Houston commuters currently has a commute of 45 minutes or more, although it's unclear how many of these are "super-commuters" – those who travel to the city once or twice per week and return on weekends – of which Houston has a nation-leading proportion according to one study).  As Chris Bradford reported back in March, this is precisely what is happening.  Thousands of new units are under construction in core neighborhoods, yet demand is so high that prices are soaring anyways. The study quoted earlier puts it succinctly:

"[A]t the metropolitan level, average density is principally a surrogate for city size. ... Thus densification, like polycentricity, is primarily a market response to contain or reduce otherwise high interaction costs found as cities increase in population ... rather than a cause of those travel times."

This theory doesn't imply that outwards expansion will come to a screeching halt once some magical distance from the center is reached, but that the balance of new construction will attempt to shift to the core as commutes from fringe areas begin to significantly exceed 30 minutes. For those despairing of an end to suburban sprawl, this may provide a glimmer of hope.  However, smaller cities with abundant land for expansion – for instance, Indianapolis, San Antonio or Oklahama City – ought to see comparatively less movement back toward the center, and that is reflected in the 2010 Census data (exceptions will certainly exist, though, due to other factors such as geography, demographics, transportation investments and city regulations).

None of this is exactly earth-shattering stuff, but in terms of looking at big picture issues for long-term urban growth, the importance of time preferences shouldn't be overlooked.

Other reading:
--Old but still relevant: Housing and the Journey to Work in U.S. Cities (Observing that the shift to auto commuting was a rational choice during the 1960-1990 period, and accounted for declines in average commuting times in spite of increasing commuting distances.  However, as of 2010, average commutes are up in all major cities as compared to 1990, in many cases significantly, showing the diminishing returns of this strategy over time).
--Basics on housing patterns and highway building: Suburbanization and Transportation in the Monocentric Model
--More spatial patterns: Traffic and Sprawl: When Jobs Suburbanize, Whither the Commute? (Echoing the finding that job decentralization is associated with shorter commutes, but observing that it cannot halt or reverse the trend of increasing average commutes as due to overall urban growth).

Rebranding the Alley: From Service Way to Narrow Street

[Updated 6/14.]

I've devoted several posts to critiquing the way many contemporary developments have reintroduced the alley as a design feature in new developments.  Although the so-called alley generally handles all utilitarian functions in these developments, including automobile circulation, trash pick-up and even emergency access, it is almost invariably accompanied by wide fronting streets which serve little obvious purpose.  Due to the conception of these alleys as mere service paths, designers generally pay little attention to their aesthetics.

One California reader, however, has alerted me to his New Urbanist community in the town of Hercules which has taken a somewhat different approach.  Rather than including a freestanding rear garage, this development runs the houses straight back to the alley, storing cars in a first-floor, two-car garage.  The alley side is not an afterthought, but instead is given a dignified architectural treatment appropriate to its role as the primary functional entrance to the home:    

Objections are often raised that new narrow streets are impossible under today's regulatory regime of fire codes and inflexible functional classification schemes, but examples such as these seem to disprove that notion.  The paved area plus flanking concrete drainage channels is 20 feet wide, conforming with the right-of-way requirements of the International Fire Code and the National Fire Protection Association. Here's another example from nearby Richmond, California, again showing an "alley" behind fronting streets, but with attached dwellings:

There is adequate room for a car to pass a stopped vehicle, should the need arise.  This should happen rarely, though, since as the street sign and painted red line show, parking is not permitted along these alleys.  Why should the city hand over a large portion of the public right of way for private car storage, after all?  And if the street is not to be used for car storage, why make it any wider than necessary?  Without on-street parking, fears of parking spillover due to new development, a frequent objection to infill densification, might dissolve.

Removing the fronting streets is not the obstacle it might seem, either.  As I wrote about last year, some new suburban developments have already adopted this design in its essentials, but with lingering confusion about how to designate the "front" and "back" of properties, and how to accommodate private outdoor space.  The resulting streetscapes are much less urban and appearance-wise far inferior to the Hercules development, which, regardless of any other failings, is helping to put the pieces in place for the conceptual transition between alley and narrow street.  The challenge of the wall of garage doors can be overcome by changing the parking configuration, lessening or eliminating parking requirements, through clever design or even by building a common underground garage.

These design changes alone, of course, are only one step among many others than are needed, including a focus on transit, a more realistic attitude toward accommodating non-residential uses, and  a city committed to properly integrating the street network with the city at large.  Even taken by themselves, however, the changes would presumably be welcome to both developers and municipalities, and have the potential to yield urban densities even using the single-family detached housing format.

Finally, one other Richmond-area development has in fact employed alley-style streets as the sole access routes for certain houses (shown below).  As a result, these paved ways – rebranded as "courts," rather than alleys – have individualized names, giving them the status of proper city streets:  

The street view along one of these courts, again showing something more than a purely utilitarian architectural treatment (note the balcony at left):

There is still a reluctance to consider these narrow streets as the territory of pedestrians as well as cars, even though there is undoubtedly very little traffic.  Instead, a pedestrian pathway runs along the opposite side of the houses on the right.  Still, the design breakthrough of separating these alleys from total dependance on corresponding fronting streets, and the conceptual breakthrough of giving the alleys official names, are noteworthy achievements here.

Related posts:
Suburban Follies: The Rear Alleyway
Suburban Follies Quick Update

Transit Types and Modal Share: More Numbers

I've put together three additional charts, correlating per capita highways, heavy rail and light rail with commuting transit share for an expanded list of thirty American cities, in the hope that these statistics might add to and shed light on the results from the previous post.  Although the first two charts are probably in line with most expectations, the third, for light rail, may come as somewhat of a surprise.

First, here are highway lane miles per capita plotted against commuting transit modal share, using highway and population data from the Federal Highway Administration (h/t Walkable DFW):

As might be expected, there is a correlation between decreasing freeway lane miles and increasing transit modal share, but it is not strong.  Eliminate the three outliers of Kansas City, St. Louis and New York, and even that modest correlation is cut in half.

By contrast, the correlation between transit share and kilometers of heavy rail is much more robust.  For this chart, only urban heavy rail systems, such as subways, were included. Regional commuter rail, such as New York's Long Island Railroad or San Diego's Coaster line, is excluded to keep the comparison consistent:

Eliminating the cities with no heavy rail from the sample reduces the correlation to an R2 value of .17.

Last, and perhaps most surprising, are the light rail numbers.  Regardless of whether cities with no light rail are included, there is a statistically insignificant (but consistently negative) correlation between light rail kilometers per capita and transit share.  The United States seems to be flush with cities with substantial light rail networks and low transit modal share.  The exceptions are the cities which also have heavy rail networks (SF, Philadelphia and Boston).  Three other heavy hitters have no light rail, although all had systems in the past (NYC, DC and Chicago):

Two cities with no or essentially no light rail at all, Milwaukee and Detroit, surpass numerous other cities with extensive networks.  Portland's extensive system, more than five times larger than Seattle's on a per capita basis, has only earned it 60 percent of that city's modal share (arguably, Seattle should be given an even lower value on this chart, as the King County branch of its light rail system opened more than halfway through 2009, the year measured by the ACS in computing modal share data).

Are there any further points to glean here?  Certainly no correlation shown here approaches the correlation of urban population density to modal share that I provided in the previous post, which remains by far the strongest correlate of transit use. The role of bus networks is obviously huge, but I wasn't able to find a single metric useful in making cross comparisons that was available for all bus operators (routes per capita is a possibility, but there is no way of knowing the length of these routes in most cases.  I might add it in later anyways, if I can locate the data for it). Commuter rail would introduce more complexities.

Chart data are below.  Numbers in the last three columns show miles and kilometers per capita (per 1,000 or 100,000).  Note that modal share data are for cities, rather than the greater urban areas from which population figures used to compute per capita highway and rail figures are drawn, but are more closely tied to the types of urban transit systems I have included.  You are welcome to reuse this information, but no guarantee of accuracy is offered.  Contact me if you'd like the excel file.

Sources:
DOT: Highway Statistics 2007
2009 ACS via The Transport Politic
Wikipedia (for light and heavy rail statistics)

Related posts: 
Chris Bradford: The association between density and mode of commute. Chris finds that standard density is weakly predictive of transit modal share, but weighted, or perceived, density is strongly correlated with it.
Laurence Aurbach: Fun with Density and Transit Statistics.

Density and Transit: Some Numbers

Note: Updated 5/29 with an expanded list of cities.  This has slightly weakened each of the correlations.

I've posted on density-related issues several times, including the relationship between height and density, building footprints and density, and street width and density, but I haven't said much about density and transit use.  Cap'n Transit has, though, recently writing a series of posts skeptical of the notion that certain areas "don't have the population density to support transit."

I think the Cap'n is right that this statement, as it's phrased, isn't all that helpful in clarifying the issue.  Instead, in all or virtually all cities, the presence of public transit is a matter of political will – in low-density Arlington, TX, the largest American city without public transportation, only political opposition has stood in the way of the establishment of a bus network. Whether the system would be profitable is presumably only one of several factors in the debate.  Below some point of total population or density, we might be confident in saying the economics would forbid any political consideration of public transportation, but that line is not a bright one.  Private mass transit is more directly tied to profitability, but this in turn is affected by political choices, both toward the private operators themselves and to competing forms of transit (which, as Cap'n has pointed out, includes cars).

A more interesting question, to me, is whether we can say anything more broadly about the relationship between density, both of population and jobs, and transit modal share. The chart I've put together below plots the residential density of a group of the largest American cities, measured by urban area, against commuting transit modal share, using data from the 2009 ACS (raw data is further down):

While there are a few transit overachievers, notably Portland, Seattle, Boston and Washington D.C., the correlation is overall very strong.  No city with an overall density of less than 4,000 per square mile, and there are many, has broken a 10 percent commuting modal share.  The most notable outlier, Miami, may possibly be explained by examining its CBD employment density and job concentration in the charts below.
  
Here is the same exercise, but using the employment density of each city's central business district (in thousands per square mile) in place of residential density:

A similar correlation is there, but it is not quite as strong.  Cities can build equally dense downtown employment districts with almost any level of transit share – cf. Dallas (4), Los Angeles (11), Philadelphia (25) and San Francisco (32).  Again, however, those cities with lower employment densities have difficulty attaining higher mode shares (the cutoff point appears to be around 100,000 jobs per square mi.).


Finally, a comparison substituting employment centralization (proportion of all jobs in the metro area located in the CBD) for employment density:

The correlation is by far the weakest here – a dense, polycentric city can achieve reasonably transit high modal share – but highly monocentric cities appear to have an easier time boosting transit use. Low density monocentric cities fare poorly. (Interestingly, Washington DC, much criticized for the economic effects of its height limit, which has allegedly driven jobs outward to edge cities such as Tyson's Corner, is the second most job-centralized of large American cities. High-rise Dallas and Atlanta, by contrast, are heavily decentralized).

The lesson here seems to be that, when it comes to increasing the share of city residents using transit residential density is a key, if not the key, factor.  It may also increase the financial viability of the transit service, although it is possible that mode share and transit expenses are also closely correlated.  That, and some other possible comparisons, will have to wait for another post. 

Employment data from 2000 Demographia study; 

mode share data from 2009 ACS via The Transport Politic.

Bikes, Transit and Traditional Urbanism

Back in 2010, Nathan Lewis published what is one of the few pro-urban critiques of transportational bicycling available on the net. In the piece, he calls into question the assumption that increasing rates of urban bicycling, or increased provision for bicycle infrastructure, are necessarily beneficial for pedestrian-centric traditional urbanism.

One of the greatest dangers, Nathan writes, is that a city consciously designs itself around the bicycle, while neglecting the needs of people on foot. It is not a groundless concern: the blog Half the Fun critiques (with photos) the Dutch city of Houten, which was designed entirely around bicycles, for having "lost sight of the forest for the trees" by its focus on cycling rather than on "creat[ing] better, more livable communities."

The article concedes that "more bikes on the road would lessen just about every transportation problem you can

A growing storage problem: bicycle parking by a transit
station, Copenhagen (Leif Jørgensen).

think of," but I think that is true only where a new bike trip replaces a trip by car. Replacing a walking trip or a transit trip is less obviously beneficial from a citywide perspective, since the bicycle introduces potential conflicts with pedestrians and presents parking issues that differ only in scale from those of cars. A review of European transportation initiatives, however, shows a frequent focus on increasing bicycle use and modal share, rather than simply decreasing automobile share, with the unstated assumption that the added bicycle riders will be drawn largely or exclusively from the pool of car drivers.*

That may not always be the case. Although evidence isn’t abundant, one study in the Danish city of Odense found that, during the mid to late-1990s, although the share of all trips taken by bicycles rose from 22.5 to 24.6%, this was accompanied by decline in the share of public transportation from 8.2 to 6.6%. By contrast, in Portland, a recent increase in bicycling share appears to have been drawn at least as much from the driving population as from transit riders. This question of whether increased funding for bike infrastructure indirectly results in the decline or stagnation of a city's own transit ridership should be of interest to municipal transportation agencies.

Based on commuting modal share statistics, it is not always easy to tell the extent to which new cycling trips have displaced transit and walking trips, car trips or both. Consider the dramatically different commuting mode shares of four European cities, each of which has taken a somewhat different approach toward transportation, while all having near-identical rates of car commuting:
 

On first glance, it's not easy to explain the differences between cities like Copenhagen and Vienna. Both are national capitals of similarly sized countries with similar metro populations and mass transit networks that include extensive subway systems.  Vienna is denser than Copenhagen, which may in part explain the much higher walking share, but we see that despite Copenhagen's immense pro-cycling efforts (so extensive that they have given their name to a popular pro-bicycling movement), its car share is only slightly less.  Vienna, by contrast, has a much more modest goal of increasing bike share to eight percent, but has primarily focused on improving transit share.

In fact, the results over time for both cities show that transit share hasn't budged, even though Copenhagen's metro entered service in the middle of the time period below (Vienna's U-Bahn has undergone recent expansion but much of the system, which opened in 1976, predates 1993).  Is it possible that pro-cycling efforts siphoned off potential transit riders in Copenhagen?  And if so, was this beneficial for the city?

Vienna street: pedestrian-centric design,
not friendly to bikes (Flickr/PaulLamere)

Vienna met its own earlier cycling share goals, but even more significant was the increase in walking share, which far exceeded expectations even though walking is hardly mentioned in Vienna's 1993 transport agenda.  What Vienna had in abundance, after all, was dense, traditional urbanism – the ultimate pedestrian infrastructure.  Through restrictions on cars, the city streets again became pleasant places to be, inducing walking far more than expected.  Copenhagen implemented similar policies, yet saw a decline in its already low walk share. 

Still, it is difficult to be too critical of increased bicycle use.  By the standard of almost any American city, Copenhagen's achievement in reducing modal share for cars is remarkable.  The bicycle, too, is an almost magical technology, the sole transportation method devised by man which has improved on the energy efficiency of walking, and one which expands the range of choices for city dwellers.

But is it enough to consider the impact of bikes on the natural environment? Shouldn't their cumulative impact on the urban environment be considered as well?  This will involve issues not only of parking ever-larger numbers of bikes as their popularity grows, but of compatibility with walking and mass transit, and of a city's vision for the interaction between the various modes of urban transport.

One final mode share chart, for New York:

The cycling share may seem surprisingly low, but consider what a bicycle is worth in the city: although it may improve mobility as compared to walking alone, it also essentially locks the bicyclist out of New York's entire public and private transit system (MTA and taxi service), all of which is implicitly designed around the person on foot (sure, there is the Metrobike and other folding bikes, but their appeal is limited and practical difficulties remain).  With the transit system running 24 hours a day, even the ready availability of the bicycle ceases to be an advantage.

All this suggests that as a city's mass transit system improves its frequency, coverage and hours, the value of a bicycle for urban mobility should decrease until, in the case of New York, it reaches close to zero for many neighborhoods.  There's room for difference of opinion, but I think this must be seen as a good thing.  Bikes can be an excellent transit gap-filler, in limited number, but may not be as well suited to being the central element of a transit strategy.


*See e.g. Copenhagen ("it is municipal policy that cycling mode share should go up to 40% by 2012 and 50% in 2015"); Groningen (city "promot[es] cycling as the main mode of transportation" with "vast expansion of the cycle network"); Charter of Brussels: (cities pledging to "set of target of at least 15% for the share of cycling in the modal split of trips for 2020").