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. For any given amount of travel time, and assuming identical roads and drivers, 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 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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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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