Category Archives: Thoroughfares

Functional Classification and Safety Statistics

Which types of urban roadways are the most dangerous? Major arterial or minor arterial? Freeway or local street? The field of motor-vehicle safety statistics is more than a century old. One would expect such a mature field to have settled the basic question of the best way to report fatality risk. But that’s not the case, and what’s more, the most common method of reporting risk may be mathematically invalid. We may be getting an incorrect understanding of the types of roadways that have the highest fatality risk.

Engineers and traffic officials often say freeways are the safest roadways and local streets are the most dangerous. That fits the view of mainstream engineering, which points out that local streets have the closest spacing of driveways and intersections, and therefore the densest occurrence of “conflict points” where crashes can happen.  Freeways and expressways have strictly controlled access in order to eliminate intersections and driveways.

That view of safety is one of the main justifications for the mandated federal policy called functional classification. The goal of functional classification is to channel a large percentage of traffic onto a small percentage of roadways, specifically freeways, highways, and arterial roads. Those roadway types are the most elaborate and designed to carry high volumes of fast traffic, often with full or partial access control. Functional classification has been very successful at achieving its basic goal of channeling and concentrating traffic.


Chart 1: Distribution of urban traffic and urban roadway miles. The chart shows the United States’ skewed pattern of travel, which is largely the product of hierarchical and dendritic roadway layouts supported by functional classification.

Chart 1 shows how functional classification sorts roadways into a range of types. The nation’s skewed distribution of traffic is apparent. Seventy-seven percent of traffic is carried by eighteen percent of roadways — by the so-called “higher-class” freeways, expressways, and major arterials. Traffic in the official statistics is reported as vehicle miles traveled (VMT). National VMT is the sum of annual state estimates, which are based on gasoline consumption, traffic counters, personal travel questionnaires, and other methods.

To compare the safety of roadways, engineers and administrators adjust the raw statistics to account for exposure. The Federal Highway Administration (and indeed the entire mainstream traffic engineering field) does that by dividing the number of fatalities by VMT. In other words, they use travel mileage as the measure of risk exposure. The chart based on that metric looks like this:


Chart 2: Fatalities per VMT by functional class

Chart 2 confirms the mainstream view. Interstates are the safest facilities and local streets are the most dangerous. But is that view truthful? Actually, the official statistics contain a critical logical flaw. VMT (that is, travel distance) is the product of speed and time. Therefore speed is fundamentally intertwined in the fatalities-per-VMT metric. The way the variables are related produces the following outcome. All else being equal, the fatalities-per-VMT metric will always report that faster speed is safer (see Appendix 1).

But that’s opposite to basic physics and everything we know from traffic safety research. Faster speed is more dangerous on most roadways, and it’s certainly more dangerous on full-access urban streets and arterials. Does a better metric exist?

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A Computerized Method to Judge Streetscape Quality

Architects and urban designers have debated streetscapes for many decades. As early as 1889, the Austrian architect Camillo Sitte wrote of the importance of continuous, connected building facades that lined streets and defined their form. Beautiful public spaces have a sense of enclosure, said Sitte; they feel like outdoor rooms. A few years later the German architect and planner Josef Stübben covered the height-to-width proportion of streets in his handbook of city planning.

In the 1920s and 30s the architect Le Corbusier issued several emphatic rejections of such ideas. “DEATH TO THE STREET,” he thundered, decrying the traditional streetscape as dangerous, unhealthy, chaotic, and congested. The ideas promulgated by Corbusier and other modernist planners were in tune with the needs of the motor industry: dispersed towers, wide buffers, and large highways. The design of new highways and arterials in cities came to resemble the auto-oriented visions of modernist planners.

The chorus of voices defending traditional streetscapes snowballed during the 1960s-1990s. Hundreds of new streetscapes were built based on traditional patterns. Infill development re-established traditional streetscapes in existing city districts. Today one can find both patterns actively promoted in all cities of the world. The debate continues as vigorously, and sometimes as contentiously, as ever.

One condition keeping the debate alive is the various arguments, theories, and polemics lack solid empirical support. Many designers and engineers have a gut feeling about the best way to design streetscapes but statistical evidence is thin. Several streetscape surveys have been developed, and researchers have used them to carry out a few studies. The surveys require teams that manually evaluate each street segment using checklists of tens or hundreds of factors. The process is laborious, slow, and expensive; and as a result, the studies to date have been limited and somewhat inconsistent.

That’s why a thesis by Chester W. Harvey titled “Measuring Streetscape Design for Livability Using Spatial Data and Methods” (2014) is an important advance in the urban design field. Harvey developed a computerized method to evaluate the essential form of streetscapes. He tested the results against a survey of streetscape appeal and found significant relationships. The theories and speculations of the past 125 years have finally been confirmed with a large sample set: streetscapes shaped like outdoor rooms are perceived to be safer and more attractive to walk in. And that is only the beginning, as the method offers many other potential avenues of investigation.

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New Urbanism and Congestion Reduction

A Wall Street Journal article, The Hidden Toll of Traffic Jams, covered several studies that linked vehicle exhaust with a variety of health impacts. The WSJ spun this as a congestion problem, even though it mentioned only one study that investigated congestion (at a toll plaza on Interstate 95). The WSJ also called the redesign of Times Square a congestion reduction measure, but in reality the congestion impacts have been mixed and travel times have increased on some routes. The Times Square redesign primarily benefits livability and pedestrian safety and comfort.

Maybe this goes without saying, but I think new urbanism has to be careful about the kind of congestion relief it advocates. There are some street network improvements that help increase the walkability and livability of an area. There are other types of construction that mostly help cars speed faster and increase sprawl.

The former can include a fine-grain mesh of small blocks, eliminating curb cuts for driveways, and intelligent systems like predictive navigation. The latter can include roadway widening, flyovers, and new freeways. There are also some measures that can go either way, like tunnels, intersection improvements, and traffic signal coordination. Depending on the design and context, these can contribute to livability or make a thoroughfare more pedestrian-hostile.

Hybrid vehicles like the Prius turn the congestion equation upside down: at lower speeds they rely more on electric propulsion, which reduces gas consumption, emissions, and health impacts. Plug-in hybrids and battery electric cars do even better. From the point of view of street-level vehicle pollution, these cars are the best thing since unleaded gasoline.

Entry limits for entire urban districts are the most direct way to reduce congestion, but outside of historic European city centers there doesn’t seem to be much appetite for doing this. Congestion tolls/fees for urban districts can be effective, but they are regressive, and the political process gets hung up on the costs and discounts the benefits. Other transportation demand measures can help, and the framework and metrics used to identify congestion are critical. Todd Litman writes:

… increased development density tends to increase congestion measured as roadway level-of-service or delay per vehicle trip, since more trips tend to be generated per acre. From this perspective, Smart Growth tends to be harmful and sprawl tends to be helpful for reducing congestion problems. However, higher density tends to increase land use Accessibility and Transportation Options, resulting in shorter trip distances and shifts to alternative modes such as walking and public transit. Although streets in higher density urban areas may experience more level-of-service E or F, implying serious congestion problems, urban residents spend less time delayed by congestion because they have closer destinations and better travel options. As a result, per capita (as opposed to per-vehicle trip or per-driver) congestion delay tends to be greater in lower-density, automobile-dependent suburban areas such as Los Angeles and Houston than in higher-density urban areas such as New York and San Francisco, because low-density areas have more per-capita vehicle mileage.

Congestion Reduction Strategies, Victoria Transport Policy Institute

New urbanists have to be informed and nuanced when navigating the congestion debate, because it easily can be turned to favor sprawl.

Below the fold: links about the level-of-service measure and its reform.

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The Power of Intersection Density

Intersection density is the number of intersections in an area. It corresponds closely to block size — the greater the intersection density, the smaller the blocks. Small blocks make a neighborhood walkable. This diagram shows three street layouts — extremely walkable, moderately walkable, and unwalkable — with their counts of intersections per square mile:

Intersection density makes surprising news in a study by the formidable academic duo of Reid Ewing and Robert Cervero. They have published Travel and the Built Environment: A Meta-Analysis in the Summer 2010 issue of the Journal of the American Planning Association.

As the title notes, the study is a meta-analysis: a study of 50 other studies about travel and the built environment. The authors look at the results from each of the 50 studies, and then pool all of those results into ten built environment measurements, including intersection density.

Their findings? Of all the built environment measurements, intersection density has the largest effect on walking — more than population density, distance to a store, distance to a transit stop, or jobs within one mile. Intersection density also has large effects on transit use and the amount of driving. The authors comment,

This is surprising, given the emphasis in the qualitative literature on density and diversity, and the relatively limited attention paid to design.

In other words, intersection density is the most important factor for walking and one of the most important factors for increasing transit use and reducing miles driven, but gets relatively little attention in research and in public policy.

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Towards a Functional Classification Replacement (Part Three)

Download a print version of this essay.

Part One of this essay covers the background, characteristics and drawbacks of functional classification, and evaluates some of the leading alternatives. Part Two continues by proposing a replacement, a sustainable transportation network classification, covering the block-scale and neighborhood-scale relationships. Part Three concludes by covering the city-scale relationship and the congestion-related impacts of a sustainable network.

City Scale

The ideal pattern of regional growth has been debated at least since the 19th century. In the 1960s and 70s the focus of the debate sharpened on efficiency and sustainability, and the “Compact City” was suggested to be the ideal. The Compact City redirects all growth into a single urban core, maximizing density while minimizing the consumption of farms, forests and agricultural land. It explicitly counteracted the dominant trend of decentralized suburban sprawl.

Some of the benefits of the Compact City idea have been confirmed by researchers. Cities with higher density and more compact form have much less per capita driving (Newman and Kenworthy, 1999). In existing cities, the trend of sprawling suburban growth causes an explosion in the amount of auto driving; a policy of refocusing growth, mixed use and transit in the urban core will halt that explosion and slightly reduce the amount of driving (Simmonds and Coombe, 2000).

Analysis of city-scale development patterns shows that focusing growth on high-capacity transit nodes will have the greatest CO2 reduction effect. Image credit: Eliot Allen, “Cool Spots”

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Towards a Functional Classification Replacement (Part Two)

Download a print version of this essay.

Part One of this essay covers the background, characteristics and drawbacks of functional classification, and evaluates some of the leading alternatives. Part Two continues by proposing a replacement, a sustainable transportation network classification, covering the block-scale and neighborhood-scale relationships. Part Three concludes by covering the city-scale relationship and the congestion-related impacts of a sustainable network.

A sustainable network classification ideally will do several things.

  • Actively encourage sustainability (as defined previously in the sustainable transport section); do not support unsustainable network patterns and operations.
  • Be concise, easy to remember and easy to explain.
  • Address a range of scales, a range that is at least as wide as that covered by functional classification.
  • Incorporate advanced knowledge about network function and best practices in network planning.

To reach these goals, a sustainable network classification is proposed. The classification has three primary relationships, each applying to a different scale. The three scales are block scale, neighborhood scale, and city scale. This allows each relationship to focus on the factors most relevant to its scale, without unnecessarily confusing factors from different scales or combining them inappropriately.

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Towards a Functional Classification Replacement (Part One)

Download a print version of this essay.

Every field has its foundational working concepts and the field of traffic engineering is no exception. It has a concept called functional classification, which is the core, guiding idea underlying the roadway system of the United States and many other nations. Functional classification is the conceptual foundation of the auto-dependent built environments where most Americans live.

The primary vision of functional classification is moving more and more cars at faster and faster speeds. This has certain benefits, but also a wide range of disastrous consequences for the built environment and the people who live in it. Hundreds, possibly thousands of reform-minded transportation planners and engineers have determined that the roadway functional classification system should be replaced.

It should be replaced by guiding concepts that support a more efficient, safer, less-polluting transportation system – concepts that support a wider range of choices for neighborhood living and daily travel. What factors should be considered when formulating a sustainable transportation system? What proposals have already been made?

Part One of this essay explores those questions. Part Two continues by proposing a replacement, a sustainable transportation network classification, covering the block-scale and neighborhood-scale relationships. Part Three concludes by covering the city-scale relationship and the congestion-related impacts of a sustainable network.

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Two Connectivity Studies for 2008

At the CNU Transportation Summit 2008, two presentations were especially consequential for the study of street networks. Both reported preliminary findings about the public safety implications of street connectivity. The results deserve attention from planners and transportation engineers.

Norman Garrick and Wesley Marshall, of the University of Connecticut’s Center for Transportation and Urban Planning, investigated the relationships between connectivity, network configuration, density, severe vehicle crashes, and mode choice. Matt Magnasco of the Charlotte (N.C.) Department of Transportation, studied the effect of connectivity on fire station service area and capital facilities planning.

In the extended entry, summary descriptions of both presentations.

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Performance Parking in DC

Market-rate parking is going forward in the new Washington D.C. ballpark district, and the pilot plan has received a good amount of coverage lately in the local press. has a roundup of links: page one and page two.

Here’s JD’s parking map page, which says: “On all streets shown in the color red, DDOT will install new multi-space meters or modify the times and prices on traditional existing meters. Multi-space meters will be programmed with rates that vary according by day and length of parking stay. These rates will be aimed at encouraging parking turnover and limiting vehicles squatting on commercial spaces.”

Why is this good news for D.C.? I previously discussed market rate parking, its justification and benefits, and a trailblazing implementation in California, in Redwood City’s Free-Market Parking Meters. If D.C.’s pilot plan is implemented as well as Redwood City’s, then the ballpark district should see reduced traffic congestion and pollution, scarce parking allocated in a more convenient and efficient way, more income for streetscape improvements, an improved pedestrian environment and therefore more business for local shops.

And who doesn’t love a solar-powered parking meter? (Okay, that was a rhetorical question.)

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Connectivity Part 7: Crash Safety

This is part 7 of a series. See also IntroductionHistorical BackgroundLatter Half of the 20th CenturyNeighborhood WalkingNeighborhood CrimeVehicle Miles and Traffic

High-connectivity thoroughfare patterns are the foundation of walkable neighborhoods and districts. While that fact was self-evident for most of human history, it was lost or ignored by land development and transportation professionals in the post-WWII era. Today, even as the numerous benefits of high connectivity are becoming increasingly recognized, the auto-centric standards of yesteryear remain firmly dominant, particularly in the field of traffic engineering.

Traffic engineering standards discourage small blocks and frequent intersections because of safety concerns. Many jurisdictions have codified those standards into their planning regulations for suburban construction, with the result that most new development consists of isolated pods depending from highway-size arterials. It is a profoundly auto-oriented pattern that is hostile to pedestrians and bicyclists. It creates the typical suburban landscape of separated, isolated pockets instead of the continuous fabric of streets and blocks that is characteristic of efficient, walkable towns and cities.

Are the safety objections against high connectivity grounded in reality? Several investigations suggest that the position promulgated by mainstream traffic engineering standards can be legitimately questioned. Because this is a new and somewhat revolutionary avenue of thought, formal statistical support is just beginning to be compiled. The initial round of studies has found that livable, walkable neighborhoods with well-connected streets are no less safe, and in some respects are safer, than the standard suburban template.

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Connectivity Part 6: Vehicle Miles and Traffic

This is part 6 of a series. See also IntroductionHistorical BackgroundLatter Half of the 20th CenturyNeighborhood WalkingNeighborhood CrimeCrash Safety

Automobiles have brought many benefits to society, and at the same time there are many good reasons to reduce the number of vehicle miles traveled (VMT) each year. Thoroughfare connectivity is part of the strategy that reduces VMT; it is a necessary, but by itself insufficient, component of walkable environments. Also, well-connected thoroughfares can reduce traffic congestion by providing alternate travel routes, and can speed emergency services by providing more direct routes and better accessibility.

There’s a nascent movement to advance better-connected thoroughfares, but meanwhile the inertia of preexisting standards and regulations discourages or prohibits high connectivity for most new construction. In addition, NIMBY residents are frequently opposed to new street connections. Future trends in energy, environmental and cultural conditions, however, could lead to walkable, well-connected thoroughfare layouts becoming more customary.

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Redwood City’s Free-Market Parking Meters

At first glance the notion of free-market parking meters seems impossibly arcane. But as Donald Shoup pointed out in a recent NY Times editorial, “cruising for curb parking generates about 30 percent of the traffic in central business districts.” Shoup studied Westwood Village, next to the UCLA campus, and found that drivers searching for curb parking created 950,000 excess vehicle miles of travel per year. That’s equivalent to 38 trips around the earth, taking place in just one retail district in L.A.

Shoup calls the impacts of parking space cruising “astonishing,” and he’s right. The unnecessary traffic congestion hurts downtown businesses and activities. The extra miles traveled waste gasoline and generate pollution. If curb parking could somehow be freed up so that it was always easy to find a space, then that extra waste and pollution could be eliminated.

One solution is free-market parking. Set parking meter prices so that 85% of spaces are occupied and 15% are open at any given moment. This idea has been getting more attention lately, and Redwood City, CA is the locality that has put the most advanced implementation into action.

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Pringle Creek’s Porous Pavement

On March 26, Pringle Creek in Salem, OR was named Land Development of the Year by the NAHB’s National Green Building Awards. The designation is a new category for the Green Building Awards, and Pringle Creek is the first development to win it.

Pringle Creek is pursuing a full suite of sustainability practices. One of the more experimental is the system of porous streets that capture and purify stormwater runoff. The developers call it an “innovative storm water management system utilizing the largest community porous asphalt and concrete road system in North America.”

Here’s a cross-section of the street design:

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Connectivity Part 5: Neighborhood Crime

This is part 5 of a series. See also IntroductionHistorical BackgroundLatter Half of the 20th CenturyNeighborhood WalkingVehicle Miles and TrafficCrash Safety

In America for a long while now, the conventional wisdom has been that isolation equals safety. In that viewpoint, one achieves maximum safety in a home that is hidden from public view, located on an isolated cul-de-sac, placed in a walled, gated community, and removed from the center city to the far exurban fringes of the metro area.

But does that model provide the best security in all situations? Or does designing for isolation sometimes provide an illusion of security?

Burglaries and street crime result from a nexus of factors, including demographics, individual psychology, and environment. Design plays a role as well. The conventional assumption is that greater isolation protects against crime, but researchers are finding that long, isolated cul de sacs with homes that are not visible from the street have higher crime rates. Crime researchers recommend breaks in the street network only under certain limited conditions.

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Connectivity Part 4: Neighborhood Walking

This is part 4 of a series. See also IntroductionHistorical BackgroundLatter Half of the 20th CenturyNeighborhood CrimeVehicle Miles and TrafficCrash Safety

A walkable neighborhood isn’t walkable unless it has a well-connected thoroughfare network. A well-connected network, composed of direct, convenient routes, is one of the key ingredients of walkability. Well-connected neighborhoods have a host of advantages for residents and for the greater community.

A large and growing collection of research is finding that street connectivity is associated with more walking, less driving, greater safety, less crime, better physical fitness, and fewer per capita emissions. This post reviews the research on neighborhood-scale relationships between connectivity and walking.

Connectivity on the neighborhood scale is about connectivity within neighborhoods. It’s about the routes and connections from building to building, from lot to lot, and from block to block. For more about the key ingredients of walkability, see the frequently asked questions page.

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