Path dependence

If you don’t know where you’re going, any path will get you there.

Path dependence is the idea that where we are today depends critically on where we were yesterday. Some systems are path independent, those that have a single unique equilibrium. Finding the solutions to some math problems is independent of where you start, as long as you follow a particular algorithm.

However, most systems we deal with on a daily basis have some characteristics of path dependence. Where you live might depend on what job you took, which depends on what your previous job was and where you went to school, and a different decision anywhere along the way would change today’s position.

Nowhere is this more true than transportation. On the one hand, it is obvious that certain locations were destined to be important cities because of significant natural advantages. New York has a deep harbor at the confluence of major navigable river. Chicago is at the pivot point between vast agricultural lands to the Northwest and the shortest land path to the East Coast. It was natural railroads would flow through the point on the map we now call Chicago.

On the other hand, many city sites that were selected for natural advantages in one technological era (The Romans selected London and the Dutch and English chose New York in large part for their capability as ports), remain important even after that technology becomes obsolete. With the logistics revolution and the new dominance of container shipping, London’s shipping has moved northeast to Felixstowe as large container ships cannot easily ply the Thames, while New York’s shipping has migrated to the wide open spaces of New Jersey.

The one-time advantages result in a set of complementary investments and inter-related decisions that take on a life of their own. Because of local trading advantages, commodities markets, banks, insurers, and other related organizations located nearby. A critical mass of those institutions felt no need to migrate just because their initial raison d’être vanished. While a building is under construction, temporary framing will often be used until the more permanent structure is erected. Once the final building can stand on its own, the falsework is dismantled. In a sense, everything is falsework for what comes after.

This kind of mutual complementarity happens repeatedly in transportation. Airplanes are the perfect example of mobile capital. If Amalgamated Airlines no longer wants to serve a particular city pair, the airplane can easily be redeployed elsewhere. Yet 80 years into the commercial aviation industry, airlines today serve mostly the same hubs their predecessors did on the Airmail routes of the 1930s. American Airlines is still in Dallas, United in Chicago, Delta (Northwest) in Minneapolis, and so on.

While very few decisions are completely irreversible, transportation decisions come close. Where we place a right-of-way, or an airport will explain where that facility will be decades, or even centuries from now.

A slight deviation from the efficient path to solve a short term problem today will cost travelers time for years to come. It is important to get the design right for the long term. (Undoubtedly this has social costs, see e.g. I-94 through the Rondo in Saint Paul).

But a slight deviation from the path will also change what the long term is. Build a bridge “here” rather than “there”, and then you will adjust all of the roads feeding into the bridge to meet it “here” (instead of “there”). And then land will be developed along the road to “here” to take advantage of the newly created accessibility, properties will be platted, buildings will be built, travel and trade patterns established, and other critical dependencies will come to assume that the bridge is “here”. At some point, say 50 years in the future, the bridge will need to be replaced. Even if “there” was a better location than “here” initially, after five decades of adaptation, it is quite likely that “here” is better now. The whole may have been better were a different initial decision been made, given conditions at the time. Given current realities, that path must now be foregone.

In transportation we say build it right the first time, because there won’t be a second chance. And that is true. But also remember the world will adapt to whatever we do, and we cannot let the perfect be the enemy of the good.

Evaluating Evanston’s Environs

I went to Northwestern University in Evanston, Illinois to attend the excellent International Transportation Economics Association meeting (we hosted an earlier version in Minneapolis in 2009). My photos are here.

2013-07-12 at 08-06-41

Evanston, like Berkeley, is an historically independent city founded in the 19th century with a major university, connected by rail to the nearby city but that was fully swept into the neighboring larger city (Chicago, Oakland/San Francisco) metropolis with the growth the automobile-highway system. It is what we would today call a first ring suburb, currently housing about 75K people. The town is very pleasant and the weather, as with all my visits to Chicago, was atypically nice. It has a high income (PCI of ~$40K, especially considering that students are counting on future not present income when spending) which adds to its pleasantness, as it ensures it is well maintained, that there is new development, and that storefronts are filled. The homes are generally well-kept, and the University is very much the archetype of university architecture, with both a strong central hand ensuring buildings keep with the look and feel of the campus, and the optimal location along the Lake Michigan waterfront.

The city is more or less on a rectilinear grid plan, though there are several different plans in place which collide awkwardly in downtown. The disadvantage is the impaired navigability of such askew streets. The advantage is the heterogeneity of spaces that are created, allowing more interesting forms and spaces to be created, beyond the uniform grid. It still has the small-town feel, with many low-rise commercial structures and diagonal parking spaces in part of downtown.

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Further, (and though I did not go there since my meals were already covered) they have a Pret a Manger, which makes me jealous, as I have to rely on Jimmy Johns for sandwiches (Pret:Johns::Johns:Subway).

Like most college towns in the second decade of the twenty-first century, Evanston has decided to establish green lanes for bikes. Though I did not see many bicyclists using them, school is out of session, and Chicago has just started its bike-sharing program, putting it 3 years behind the Twin Cities.

Strangely, given the relative completeness of the Chicagoland freeway and rail systems, Evanston is not well connected to O’Hare, taxis must use surface streets for extended distances to get to the airport, while rail goes through the city of Chicago.

My favorite part of the airport trip was discovering a Cadillac dealership at Lincolnwood Town Center. Note, doing an Internet search, I find there is also a Lincoln dealership in Cadillac, Michigan. Anyway, “he meant Lexus, but he ain’t know it” (NSFW – language).

2013-07-12 at 08-05-37

Lessons from Exploring Evanston

  • College towns have a head start on economic success.
  • Exploiting naturally provided scenic beauty helps.
  • Airport access is relatively unimportant – most people are not going to the airport most of the time.
  • Locating at the edge of a metropolitan area allows the best of both worlds, access when wanted, isolation when desired.

Time Evolution of Twin Cities

Google Earth Engine lets you see the evolution of Landsat photos.
We did this for the Twin Cities. Go here:
Timelapse of Minneapolis – Saint Paul
Andrew notes:

I think the most amazing thing is seeing the path of the Minneapolis tornado appear in 2011

Cross-posted at Streets.MN

Grids are for squares: Three reasons to consider alternatives to rectilinear street networks

Elements of Access: Transport Planning for Engineers, Transport Engineering for Planners. By David M. Levinson, Wes Marshall, Kay Axhausen.

Just as we have cut the earth into a grid of latitude and longitude (and knowing that each “block” of 1 degree latitude by 1 degree longitude gets smaller and smaller as we approach the poles), we similarly cut our cities and rural areas into a finer mesh from that same grid. Much of this arises from the various large scale ordinance surveys that took places in the Americas, Australia, and India. There are of course grids dating much earlier, to Miletus and Mohenjo Daro among many others. Not all grids are aligned with longitude and latitude, sometimes they align with local landscape features, but most of the modern ones are. (Where grids of different alignments come together, interesting spaces are created). Not all grids are squares, most are more like rectangles.

So why should we have 90-degree rectilinear grids?

The arguments in favor are that it:

  1. simplifies construction and makes it easier to maximize the use of space in buildings,
  2. simplifies real estate by making the life of the surveyor easier,
  3. simplifies intersection management by reducing conflicts compared to a 6-way intersection,
  4. is embedded in existing property rights and so impossible to change.

We in the modern world need not be bound to the primitive tools of the early surveyor, the primitive signal timings of the 1920s traffic engineer, or the primitive construction techniques of early carpenters. And while for existing development we might be locked into existing property rights, for new developments that doesn’t follow.

The arguments against the rectilinear include that it:

  1. is among the least efficient way to connect places from a transportation perspective,
  2. reduces opportunities for interesting architecture,
  3. wastes developable space by overbuilding roads.

There are many designs for non-rectilinear street networks. Ben-Joseph and Gordon (2000) (Hexagonal Planning in Theory and Practice (Journal of Urban Design 5(3) pp.237-265)) summarize a number of the 19th and 20th century designs. Most are simple aesthetic choices, as in Canberra, the planned capital city of Australia, and don’t seem to relate to deeper urban organizational issues.
Rudolf Müller proposed The City of the Future: Hexagonal Building Concept for a New Division. Müller’s plan offsets the 60-degree streets so that they come together in 4-way rather than 6-way intersections (though they are still at 60-degrees and not bent to make 90-degree intersections). This ensures that the cells in the plan are not bisected by roads, and that they are instead hexagonal blocks. This plan loses a lot of areas to ornamental parks in the middle of streets.
The circuity increase associated with a 90-degree rather than 60-degree network is obvious. Circuity (the ratio of Euclidean to Network distance) would be minimized if roads were at 0-degree angles. The downside is that this Euclidean network where everyone traveled in a straight-line would literally “pave the earth“. Leaving aside the downsides for the environment of being so-paved, the more critical trade-off from a transportation perspective is construction costs. More roads are more expensive. So a network design trades-off travel costs accruing over time with the up-front construction and long-term maintenance costs. The optimal network design depends on the land use pattern it aims to serve. (And the land use pattern depends on the network design.) The City of Alonso or Von Thünen, with all jobs downtown merely requires a simple radial network to connect it. A polycentric or fully dispersed (homogeneous) city with everything spread uniformly across space begs for more cross-connections.

Charles Lamb’s City Plan has the streets hexsect the hexagonal cells. In this case, the blocks are really triangles.

There is a large literature on the network design problem. One useful paper: Pierre Melut and Patrick O’Sullivan (1974) A Comparison of Simple Lattice Transport Networks for a Uniform Plain, Geographical Analysis 6(2) pp. 163–173, says:

The objective is to compare construction and transport costs for triangular [60-degree], orthogonal [90-degree], and hexagonal [120-degree] regular lattices as transport networks serving a uniform, unbounded plain. The lattices are standardized so that the average distance from the elementary area to the edge is the same for each. This standardization results in equal construction costs for the three networks; thus, the comparison can be made in terms of route factors [circuity], which favors the triangular lattice over the other two.

Because the circuitous network is less efficient, more network pavement and track and vehicle mileage must be provided to enable the same amount of transportation.

This wastes spaces that could be better allocated to non-transportation purposes.
The lattice itself comprises a single level in a hierarchical system. Selected links in a lattice can be reinforced to make them faster, attracting traffic. This process of reinforcement is natural with investment rules that favor more heavily trafficked routes and explains the hierarchy of roads. If it is based on simple reinforcement of existing links rather than creation of new links, that hierarchy will not affect the topology of the network.

Ask MetaFilter has an interesting thread on Comparing perimeters of arrays of hexagons vs. squares – geometry tiling resolved . A key point is that arranging hexagons into a square-like shape has a higher perimeter than arranging squares into a square-like shape.

__    __    __    __    __
/  \__/  \__/  \__/  \__/  \
\__/  \__/  \__/  \__/  \__/
/  \__/  \__/  \__/  \__/  \
\__/  \__/  \__/  \__/  \__/
/  \__/  \__/  \__/  \__/  \
\__/  \__/  \__/  \__/  \__/
/  \__/  \__/  \__/  \__/  \
\__/  \__/  \__/  \__/  \__/
/  \__/  \__/  \__/  \__/  \
\__/  \__/  \__/  \__/  \__/
Diagram 1. Sample hex map

Jellicle wrote:

I think your problem is this – to minimize the perimeter of n hexagons, when you add each new hexagon to the previously-existing group, you have to add it in such a way that touches the most neighbors possible. You would never add a hexagon that touches only on one face if you could add it somewhere else where it touches two faces or three faces, right? If you look at diagram 1 here (which is hexes in a grid shape), you see several hexes at the four corners which touch only on two faces, while there are areas on the outer surface at the top and bottom where those hexes could be placed where they would touch on three faces instead of two. So simply moving those four corner hexes would reduce the perimeter without changing the surface area.

Yet we know the hexagon is efficient, it replicates the closest packing of circles. (Take a penny, surround it with pennies so that they are all tangent. The central penny touches six others.) Thus following the closest-packing argument, the hexagon as geometrical shape is not sufficient for efficiency, we must also arrange those shapes into an efficient pattern, in this case, something more like the Glinski Chess Board:
Much of the inspiration for thinking about hex-maps comes from the gaming community, where such maps have been used since the 1961, when a Hex map was used for the Avalon Hill game Gettysburg. It has since become a standard that is widely used to represent directions of movement in games.

So, although we talk about “grids” as being necessary for connectivity, we can get even more connectivity if we think about a variety of different geometries. It would be a shame if we got locked into grid geometries for new developments when there are so many alternatives to be had.

See also: Home is Where the Hub Is.

Travel Time Perception Errors: Causes and Consequences

Congratulations to soon to be Dr. Carlos Carrion (shown in the center of the picture, between alums Nebiyou Tilahun and Pavithra Parthasarthi), who recently defended his Ph.D. Thesis “Travel Time Perception Errors: Causes and Consequences” (a draft of which is linked). He is working as a post-doctoral researcher at MIT/SMART in Singapore.

Travel Time Perception Errors: Causes and Consequences
This research investigates the causes, and consequences behind travel time perception. Travel times are experienced. Thus, travelers estimate the travel time through their own perception. This is the underlying reason behind the mismatch between travel times as reported by a traveler (subjective travel time distribution) and travel times as measured from a device (e.g. loop detector or GPS navigation device; objective travel time distribution) in collected data. It is reasonable that the relationship between subjective travel times and objective travel times may be expressed mathematically as: Ts = To + ξ. Ts is a random variable associated with the probability density given by the subjective travel time distribution. To is a random variable associated with the probability density given by the objective travel time distribution. The variable ξ is the random perception error also associated with its own probability density. Thus, it is clear that travelers may overestimate or underestimate the measured travel times, and this is likely to influence their decisions unless E(ξ) = 0, and Var(ξ) ≈ 0. In other words, travelers are “optimizing” (i.e. executing decisions) according to their own divergent views of the objective travel time distribution.
This dissertation contributes novel results to the following areas of transportation research: travel time perception; valuation of travel time; and route choice modeling. This study presents a systematic identification of factors that lead to perception errors of travel time. In addition, the factors are related to similar factors on time perception research in psychology. These factors are included in econometric models to study their influence on travel time perception, and also identify which of these factors lead to overestimation or underestimation of travel times. These econometric models are estimated on data collected from commuters recruited from a previous research study in the Minneapolis-St. Paul region (Carrion and Levinson, 2012a, Zhu, 2010). The data (surveys, and Global Positioning System [GPS] points) consists of work trips (from home to work, and from work to home) of subjects. For these work trips, the subjects’ self-reported travel times, and the subjects’ travel times measured by GPS devices were collected. Furthermore, this dissertation provides the first empirical results that highlight the influence of perception errors in the valuation of travel time, and in the dynamic behavior of travelers’ route choices. Last but not least important, this dissertation presents the most comprehensive literature review of the value of travel time reliability written to date.

$40bn “fix it first” plan headlines Obama’s infrastructure push

I get quoted in Global Construction Review: $40bn “fix it first” plan headlines Obama’s infrastructure push

In his State of the Union address last month, US President Barack Obama proposed investing $50bn, starting right away, on the country’s transportation infrastructure.
Of that, $40bn would go toward the upgrades most urgently needed on highways, bridges, transit systems, and airports in what the White House has dubbed a “fix-it-first” policy.
“The national transportation system faces an immense backlog of state-of-good-repair projects, a reality underscored by the fact that there are nearly 70,000 structurally deficient bridges in the country today,” the White House said in a statement.
Mr Obama’s plan, which would need congressional approval, also proposes attracting private investment by pairing federal, state, and local governments with private capital, in what’s being called the “Rebuild America Partnership”.
And a third plank in the President’s infrastructure push is cutting red tape. Through a “historic modernisation of agency permitting and review regulations, procedures, and policies”, the President hopes to cut in half the duration of typical infrastructure projects.
The “fix-it-first” element of the plan received a muted welcome from Professor David M Levinson, an expert on the economics of infrastructure at the University of Minnesota.
“The priority should clearly be on repair because most of the system is built out, and we’ve had nationally declining travel over the last 10 years, so there’s not a major need for expansion nationally,” he told GCR.
The American Society of Civil Engineers (ASCE) has warned of an investment gap of $846bn in surface transportation
“The general problem is that the median age of an interstate highway link in the US is almost 50 years old now, and the expected lifespan of such links was in the order of 50 years.
“Generally most of the infrastructure that has got to be there 10 years from now is there now, and if we want it to be there ten years from now we need to fix it.”
The American Society of Civil Engineers (ASCE) has warned of an infrastructure investment gap, between now and 2020, of $846bn in surface transportation. If not addressed, says the ASCE, this shortfall will hurt the US economy.
Is $40bn enough?
“No,” Prof Levinson said. “No one really knows what’s enough. It’s about the equivalent of one year’s federal spending on roads. So it would be like adding an extra year to the decade, or 10% more over 10 years. It’s not trivial. It’s not going to solve the problem, either, but it’s a real amount of money.”
He also questioned the wisdom of infrastructure investment driven by the federal government.
“The states should be addressing this,” he said. “They can prioritise things locally, they know where the issues are, and they’re the beneficiaries.
“They know how much they need to spend locally to satisfy the local risk-reward, benefit-cost ratio. The federal government allocates things by formula and that means there’s a major inefficiency there.”

Eliminate the bi-annual time change caused by Daylight Savings Time

A new petition at asks to Eliminate the bi-annual time change caused by Daylight Savings Time :

“Daylight Savings Time is an archaic practice in our modern society.
The original reasons for the policies are no longer applicable, and the most cited reason for keeping DST (energy savings) has never been shown to be true.
Some industries still like DST (like sporting equipment retailers), but there are many more who dislike the changed hours (like television).
The real issue, however is not the later hours or extra sunlight. Studies have shown that changing the clocks is responsible for health problems (including increased heart attack and vehicular accident risks) and leads to hundreds of thousands of hours of lost productivity in workplaces across the country. Also: It’s really annoying.
We should either eliminate DST or make it the year-round standard time for the whole country.”

While our results on the traffic safety consequences are mixed (DST reduces crash rates but increases traffic) it nets that DST enhances safety. Still, I tend to agree with the petition, we could achieve the benefits by having a fixed adjustment year round. And if people want to switch behaviors relative to the clock, good for them. Me, I would eliminate timezones too, and have one global time, so I wouldn’t have to go to work until 14:00 GMT and could sleep in every day.
Huang, Arthur and David Levinson (2010) The Effects of Daylight Saving Time on Vehicle Crashes in Minnesota Journal of Safety Research 41 513-520.

Just-in-time consumption: Does the `pint of milk test’ hold water?

Cross-posted from Just-in-time consumption: Does the `pint of milk test’ hold water?

“As with stores, houses too are getting larger over the long run. New suburban homes have more space to store goods in-house. While urban residents export storage to common stores, suburban residents more likely to have second freezers, have more space to store stuff.”

Just-in-time consumption: Does the `pint of milk test’ hold water?

Just-in-time production revolutionized manufacturing, enabling both a reduction in inventories as supplies arrive only shortly before needed, and an improvement in quality as poorly made inputs are no longer stored for long periods of time, but can be quickly identified and feedback provided to the supplier. The widespread adoption of the just-in-time process is itself the product of the logistics revolution, information and communications technologies, containerization in shipping, and the modern freeway system. It has seen a concomitant change in the retail sector, which has brought about fewer and larger stores at a greater distance from the end consumer.The notion of “just-in-time consumption”, (acquisition of a good by the end consumer shortly before its use, rather than being acquired and stored for future use) though seemingly a natural mirror to the more widely used “just-in-time production” has not received the same attention. The phrase itself, only generates 212K hits in Google, (significantly higher than 2007, and of which the original version of this post rates #2) of which only a few are on-point, in comparison to over 2.23M for “just-in-time production”.Yet many goods and services are already consumed in a just-in-time manner. Most notable is energy, which is delivered on-demand to users, who no longer store coal at home for the furnace, but instead buy natural gas or electricity as needed. (The slowly vanishing home heating oil remains an exception). Other services that are provided on-demand or just-in-time include water and sewer, communications (internet, telephony, and television). What is in common about these disparate technologies is their network nature, the large infrastructure required to enable using the flows on-demand. While sewer is a continuous service for most people (those who do not have septic tanks), garbage is typically only collected periodically (e.g. once a week), and recycling less so (e.g. fortnightly).Other goods once saw regular to-the-house delivery, especially in suburban areas. Foxell [2005] writes of goods and services found in Metro-land, the idyllic north London suburbs built by the Metropolitan railway in the early twentieth century:

“This service economy is illustrated by the variety of tradesmen that called at our home: the milkman twice a day, with a horse-drawn cart; the baker once a day, with a large upright barrow on two wheels, the handles of which lifted him off the ground when going down hill; the postman thrice; the butcher’s boy by bicycle twice a week; and the grocer twice a week. Others like the coalman or the Gas, Light & Coke Co. in their steam-powered Sentinel lorry also made regular deliveries. Over a longer period, visits could be expected from the men from the Prudential [insurance], Hoover [vacuum cleaners], Singer [sewing machines] and the like – all using a service call to take the opportunity to sell new products. There was something reassuring about seeing such familiar faces and catching up with the latest gossip. In addition there were the itinerant callers such as Walls Ice Cream man on his tricycle as well as the French onion sellers, gypsies with pegs and posies, rag and bone men, tinkers [metalsmiths] and the knife-sharpeners with their pedal-driving grinding wheels.”

Today, the vast majority of those goods are not acquired at home but in stores or online. Delivery services have replaced salesmen, as the two functions (delivery and sales) are now distinct and specialized. Today’s visitors might be the post office, FedEx or UPS, and the pizza delivery boy.

Just-in-time does not require delivery to the residence, it can involve ubiquity in the placement of stores, so that they are near the end consumer. Traditionally the retail store was just that, a place where a community could store goods, and individuals could take or buy them as needed. A new model of temporary lockers may emerge to fill the gap.


Many planners would like to make the ability to acquire goods just-in-time without the use of a vehicle a normative planning standard. For instance, a report, Beyond 2010: A Holistic Approach to Road Safety in Great Britain calls for the “pint of milk test”, for all new developments, whereby a resident can get to a shop to buy a pint of milk in 10 minutes or less without getting in their car [Parliamentary Advisory Council on Transport Safety, 2007]. The idea of 10 minutes comes from people’s willingness to walk, people are less willing to walk longer distances than shorter, and 10 minutes (or one-half mile (0.8 km)) seems to be a threshold over which walking tolerance seems to drop. This distance was derived from several empirical studies, including Pushkarev and Zupan [n.d.], who showed the median walk by travelers accessing the New York subway was 0.35 mi (0.57 km), while the median walk to access commuter rail stations in suburban New Jersey was 0.5 to 0.6 mi (0.8 – 1.0 km). Results from the 1983/84 National Personal Transportation Survey reported by Unterman [1990] found shorter distances: 70 percent of Americans will walk 500 feet (0.15 km) for normal daily trips, 40 percent walk 1,000 feet (0.31 km), and only 10 percent walk a half-mile (0.8 km).The pint of milk refers to a standard quantity of a highly perishable and frequently consumed good. The objective of avoiding car use is obvious for a group advocating road safety. The pint of milk test has received some currency in England, being noted by several studies in recent years [Bennett and Morris, 2006, Marsh, 2004]. This is a particular issue in a crowded city like London, where auto ownership is lower than suburban areas, roads are more crowded, and parking more difficult even for those with a car.

The trends in retailing have been clear in the United States for a long time. Stores are over the long term getting larger and gaining larger market areas [Yim, 1990]. Small stores serving local areas have been losing market share to larger stores which bring with them economies of scale. Efforts to reverse this trend have met with resistance from retailers, consumers, and neighbors [Nelson and Niles, 1999].

Illustrating this trend, the Food Marketing Institute reports in 2011 there were 36,569 supermarkets (with $2 million in sales or more, noting the median annual sales for a supermarket was $17 million, and average size was 46,000 sq. ft. (slightly down form a 2005 peak of 48,058, indicating an increasing number of smaller markets in recent years, but nowhere near retracing the long march upward). The average number of trips per week consumers make to the supermarket was up to 2.2. (from 1.9 in 2006).

In 1930, The Great Atlantic and Pacific Tea Company, at the time the leading US supermarket, alone had 16,000 stores with a combined revenue of $1 billion (or per store revenue of $62,500 in 1930 dollars, estimated to be $754,000 today) [The Rise and Decline of the Great Atlantic and Pacific Tea company, n.d.]

Handy [1993] claims “the automobile instigated a collapse of the retail hierarchy by encouraging the growth of community and regional centers at the expense of local shops and the central business district. The result has been a cycle of dependence, in which suburban communities are designed for the automobile leaving residents little choice but to drive.”

As with stores, houses too are getting larger over the long run. New suburban homes have more space to store goods in-house. While urban residents export storage to common stores, suburban residents more likely to have second freezers, have more space to store stuff.

While the number of freezers per household in the United States is declining as second freezers are being retired and not replaced, the number of refrigerators is increasing slightly, due to households obtaining second refrigerators. [Wenzel et al., 1997]. While no immediate inference can be made about this, other trends are also at work. Total refrigerated and frozen space has not been computed, though the average size of a house’s primary refrigerator or freezer is likely increasing. Food may last longer in refrigerators than it used to due to the addition of preservatives (though the trend of increased consumption of organic foods may reverse this). Further globalization may mean that fewer goods are seasonal and need to be accumulated prior to their being out-of-season.

Persson and Bratt [2001] note that e-shopping may induce the installation of a second set of fridge/freezers per household to receive delivered goods. This additional electricity consumption has environmental consequences; already, there are 2.2 refrigerators and freezers per household in New Zealand (Roke, 2006) cited in [New Zealand Ministry for the Environment, n.d.].

If urban residents do undertake more just-in-time consumption than suburbanites both because of the higher storage costs associated with smaller houses, and the greater opportunity afforded by more stores nearby, we would expect to see this show up in the travel behavior data that is collected by urban regions.

variable Minneapolis St. Paul Remainder of Hennepin County
Year Structure Built 1926 1929 1970
Sq. Ft. 1773 1826 2152
Sq. Ft. per Person 822 755 810
Households with No Cars 5900 2800 2500

Table 1 illustrates some of the differences between the City of Minneapolis, suburban Hennepin County (Hennepin excluding the City of Minneapolis), and the City of St. Paul in neighboring Ramsey County. Residents of Minneapolis live in older houses (average year built of 1926 vs. 1970 in the suburbs) with 1773 square feet vs. 2152 in the suburbs. However because of the lower household size, city residents actually have slightly more area per person. Further Minneapolis residents are more likely to be carless.

According to the 2000/2001 Twin Cities Travel Behavior Inventory among residents of the City of Minneapolis, 12.8 percent of daily trips were for shopping 5 while for Hennepin County excluding the City of Minneapolis the number was 12.2 percent. Thus Minneapolitans devote 5 percent more of their trips to shopping than suburban Hennepin County residents.

Minneapolitans also make slightly more trips than their suburban brethren, 3.81 per day vs. 3.70 for suburban Hennepin. (The unpublished 2011 TBI will likely show significantly lower numbers here). Given the small differences and their temporal instability, it probably is unreasonable to make much of them.

The evidence supports the hypothesis that city residents who have somewhat higher accessibility (see Figure) to neighborhood stores and somewhat reduced storage space at home shop more frequently.


Broadly, there are two types of places, those that satisfy the pint of milk test, and those that don’t. Similarly, there are two kinds of people, those who care about the pint of milk test and those who don’t. The problem comes from the mismatch of those who care but live in places that are unsatisfactory. (Those who don’t care but live in places passing the test are probably okay). If self-selection is at work, these cells are not randomly distributed, but people who want to live in particular environments do so. People who prefer milk-accessible areas bid up prices in those areas, while those who are indifferent (or perhaps lactose-intolerant) move out. However, if preferences change faster than spatial structure, there may be a mismatch.

Policy that excludes mixture of residential and commercial development may also foster a mismatch.

Evidence from the Twin Cities bears on the issue (Figures 2 to 5). According to the American Housing Survey [US Census Bureau, n.d.], over 80 percent of residents in the City of Minneapolis report satisfactory neighborhood shopping within a mile of home, compared with 70 percent of those in suburban Hennepin County (Figure 2). Despite that positive assessment of shopping, suburban Hennepin residents have a better opinion of their own neighborhood than those in the City of Minneapolis (Figure 3). The problems these urbanites report in greater numbers than their suburban counterparts are noise and traffic, crime, and odors (Figure 4).





When people move, they are doing so to places they believe are better, but for all residents it is the home that is better than previous much more so than the neighborhood, and in Minneapolis, only a third rate their current neighborhood as better than their previous (in contrast to half of suburban residents) (Figure 5).

To the extent neighborhood shopping enabling just-in-time consumption of the pint of milk is important to people, cities fare better than their suburbs, but if the cost of that neighborhood shopping is other urban ills, people will make the trade-off, sacrificing access to retail to have access to quiet and congestion free, safe, and pleasantly smelling suburban environments.

Whether this is a social good is another question entirely, and depends on relative efficiency of urban goods delivery services, energy efficiency of in-store displays vs. at-home refrigeration units, and numerous other questions.


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Adapted and updated from a post on The Transportationist Nov. 8 2007.

Walkable Ice

In the absence of significant global warming, Minnesotans still need to contend with ice on the sidewalks (to be clear, in the presence of significant global warming, we would have other problems; and in the presence of significant global cooling, we would face snow and glaciers rather than freezing rain and ice).
My own house suffers this problem, despite (or because of) snow clearance, ice re-forms on the sidewalks and steps, or freezing rain falls on the cleared sidewalks, making them slick, rather than on snow-covered sidewalks, making them crunchy. Further, water drips from the house and gutters because of ice dams, and then freezes on the ground.
My alma mater, Georgia Tech, while not typically subject to much snow or ice, has many sidewalks just above steam-heat pipes, which would clear those sidewalks pretty readily in most conditions. The University of Minnesota does a pretty good job with snow clearance, all things considered, using a lot of labor and snow clearance machines in the process.
Ice clearance is hard in this freeze-melt cycle, especially when the water has no where to drain because (1) the sidewalks are convex (along either width or length), (2) the boulevards are covered in snow creating no place for run off to go and creating a source for new melted water, (3) the storm drains are covered in snow, and (4) the ground is still frozen and/or the soil above the freeze line is super-saturated.
I see a lot of attention to ice-free roads, and very little for ice-free sidewalks. This would greatly enhance walkability, reduce the likelihood of severe injury, and increase the number of pedestrians.
There are a variety of ways to address icy sidewalks:

  • Mechanical: clearing sidewalks with shovels and pick-axes and snow-bots.
  • Friction: Sand, Grit, Gravel make the ice more walkable (by increasing friction);
  • Chemical: Salt (reduces ice via melting);
  • Radiant: heated sidewalks (using a variety of techniques);
  • Protection: covered sidewalks; and

If we consider the cost of an icy sidewalk equal to the probability of a fall multiplied by the cost of a fall, multiplied by the number of people who face that probability per day, times the number of days the sidewalk is icy, we can get a sense of the amount we should invest to avoid the ice.
Let’s say I fall once a year on the ice (typical), after traveling 2.6 km * 2 times a day * 10 ice days = 52 km. My fall rate: is 1 fall per 52 km of ice.
For a house with 10 m of frontage, with 100 pedestrians a day, it gets 1 km of pedestrian traffic per day. Once every 52 icy days, it will see someone fall.
The cost of a fall is unclear, since most falls are unreported. For reported falls which require medical care, the estimate is on the order of $10,000. Let’s assume 10% of falls require medical attention, meaning the average cost per fall is $1,000.
This implies that every 52 icy days (once every 5.2 years if there are 10 icy days per year), each house with icy sidewalks imposes $1,000 in costs. In that case, if we want to minimize social costs, we should be willing to invest $19 day in effective ice clearance. This is about an hour of labor (or two hours of undergraduate labor) to operate simple machines plus some cheap (Friction or Chemical based) treatments). Unfortunately, I am unclear whether $19/day is effective.
We could add delay costs, due to people walking slower on ice, which I estimate to be about a 10% reduction in walking speed. With a travel speed typically of 1.44 m/s, we might decrease that to 1.3 m/s. So instead of the 100 pedestrians taking 7 seconds each to walk in front of the house, they are taking 7.7 seconds. That is 70 person-seconds per day, which has an economic value of (@ $15/hour) of $0.30 per day, two orders of magnitude lower than the fall costs, and so not really worth discussing further.


But can we prevent the ice from forming?
For $1000 every 5.2 years, we get $5000 for a 26 year expected life of a capital investment. If we can make a capital investment of less than $5000 to eliminate falls on our public sidewalk, it would be socially worthwhile.
The cost of heating sidewalks is about $20 per square foot (or about $215 per square meter). A 10 meter by 2 meter sidewalk is 20 meters square, giving us a cost of $4305.
We must consider operating costs, which are estimated at $.60/hour. If it is operating 240 hours per year (this is a guess, I don’t know how long it needs to operate to keep the sidewalk ice free), this is $144 year. (You might run it to melt snow, but that has fewer benefits, just avoiding shoveling, not reduced falling in this simple model, so I don’t consider that). $144 per year is $3744 over 26 years (no discounting), so is a large fraction of the capital costs.
Unfortunately, $4305+$3744 > $5000, so 100 pedestrians is not enough to justify heating. However 160 pedestrians would be a break-even point.
Covering the sidewalks (200m of roofing) could cost $80/square foot ($860/square meter). This lasts 15 years. For 20 square meters, this costs $17,200, well out of range for our residential sidewalk if the only objective is ice reduction, especially since it only lasts 15 years. It might have other benefits, such as reducing our exposure to nature and street-life though.
Policy recommendation: Use student labor to clear sidewalks with low pedestrian flows. Heat sidewalks which have high pedestrian flows. Cover sidewalks with very high pedestrian flows.
Yes, I did fall this year. This post was written between my vertical and horizontal positions, so I apologize in advance for its rushed nature.