Imagine that we used a really crazy method to establish speed limits. We could start by recording the speeds of all drivers on a given stretch of roadway. Then, without any clear evidence of what a safe speed might be, we might argue that the great majority of people drive too fast, and therefore the maximum legal speed will be set as that speed exceeded by 85 percent of drivers. Only the slowest 15 percent of drivers, in this scenario, would be considered to be driving within the legal limit.

If you have a passing familiarity with the legal framework of car culture, you will recognize the above as a simple inversion of the common 85th percentile rule used by traffic engineers throughout North America. Following this guideline, driver speeds are recorded, engineers determine the speed exceeded by only 15 per cent of the drivers, and that speed is deemed an appropriate speed limit for the given roadway. All the other drivers – 85 per cent – will then be driving within the speed limit.

Two recent books argue that the 85th percentile guideline is as arbitrary and misguided as it sounds. In There Are No Accidents, (Simon & Schuster, 2022; reviewed here last week), Jessie Singer summarizes the 85th percentile rule this way:

“Most speed limits are not based on physics or crash test expertise but simply the upper limit of what most amateur drivers feel is safe. A speed limit is the perceived safe speed of a road, not the actual risk of traveling that speed on that road.” (Singer, page 95)

Singer draws on the work of Eric Dumbaugh, who has a PhD in civil engineering and teaches urban planning at Florida Atlantic University. Dumbaugh has analyzed tens of thousands of traffic crashes in urban environments in the US. He concluded that the traffic engineering guidelines used for decades are based on false information, are often misapplied, and result in dangerous conditions on urban roadways. Absent physical evidence of what constitutes a safe driving speed, engineers simply assume that most drivers drive at a safe speed. Dumbaugh doesn’t mince words:

“Traffic engineering is a fraud discipline. It presumes knowledge on road safety that it doesn’t have and it educates people generation after generation on information that is incorrect.” (quoted by Singer, page 96)

The dangerous conditions on roadways have contributed to thirty thousand or more deaths in the US every year since 1946. But the engineers who design the roadways cannot be faulted, so long as they have applied the rules passed down to them in standard traffic engineering manuals.

Confessions of a Recovering Engineer was published by Wiley in 2021.

Similar themes are also a major focus in an excellent book by Charles Marohn Jr., Confessions of a Recovering Engineer (Wiley, 2021). Marohn was trained as a civil engineer, and for the first part of his career he worked as a traffic engineer designing what he saw at the time as “improvements” to roadways in small cities. Over time he began to question the ideas he had absorbed in his education and the guidelines that he followed in his engineering practice.

Marohn is now founder and president of Strong Towns. He has emerged as one of the most vociferous critics of the planning principles underlying American suburbia, and the design guidelines used to justify the arterial roads in those suburbs. He writes,

“The injuries and deaths, the destruction of wealth and stagnating of neighborhoods, the unfathomable backlog of maintenance costs with which most American cities struggle, are all a byproduct of the values at the heart of traffic engineering.” (Marohn, page 5)

These values are held so widely and deeply, Marohn says, that they are seldom questioned or even acknowledged. These values include :

  • “Faster speeds are better than slower speeds..
  • Access to distant locations by automobile is more important than access to local destinations by walking or biking. …
  • At intersections, minimizing delay for automobile traffic is more important than minimizing delay for people walking or biking.”-(Marohn, page 12)

Working from his own experience as a traffic engineer, Marohn explains the order in which issues are considered when designing a new or “improved” roadway. First the engineer decides on a “design speed” – a driving speed which the road should facilitate. Next to be established is the traffic volume – all the traffic typically traveling the route at present, plus all the additional traffic the engineer anticipates in the future. At that point the engineer will choose a design based on official guidelines for that design speed and that traffic volume; so long as the guidelines are followed, the design will be deemed “safe”. Finally, the engineer will estimate how much it will cost.

Marohn argues that the questions of whether traffic should move slow or fast, and whether all existing traffic should be accommodated or instead should be restricted, are not technical issues – they are questions of values, questions of public policy. Therefore, he says, issues of the desired traffic speed and desired traffic volume should be dealt with through the democratic process, with public input and with the decisions made by elected officials, not by engineering staff.

Image courtesy of Pixabay.

Some sins are forgiven

In the early days of car culture, traffic casualties happened at a far higher rate per passenger mile than they do in recent decades. Part of the improvement is due to changes in vehicle design – padded surfaces, seat belts, air bags. Part of the improvement can be attributed to what is called “forgiving design”, at least as applied on rural highways. Examples of forgiving design are gradually sloped embankments, which reduce the likelihood of rollovers if a driver veers off the road; wider lanes which lessen the chance of sideswiping; centre barriers which prevent head-on collisions; straightening of curves to improve sightlines; and removal of roadside obstacles such as large trees which an errant driver might hit.

On highways these forgiving design principles make sense, Marohn believes, but on urban arterial roads they are disastrous. He coined the word “stroad” for urban routes that combine the traffic complexity of streets with the high design speeds of inter-city roads. Stroads feature the wide lanes, cleared sightlines and levelized topography of highways, giving drivers the impression that higher speeds are safe. But stroads also have many intersections, turning vehicles, and access points for pedestrians. This means that the higher speeds are not safe, even for the drivers. And vulnerable road users – pedestrians and cyclists – often pay with their lives.

Most stroads should be converted into streets, Marohn says. “Instead of providing drivers with an illusion of safety, designers should ensure the drivers on a street feel uncomfortable when traveling at speeds that are unsafe.” (Marohn, page 43) To ensure that the mistakes of pedestrians and cyclists, and not just drivers, are forgiven, he advocates these guidelines: “Instead of widening lanes, we narrow them. Instead of smoothing curves, we tighten them. Instead of providing clear zones, we create edge friction. Instead of a design speed, we establish a target maximum travel speed.” (Marohn, page 41)

On a typical urban street, with stores, offices, schools, restaurants, and many people moving around outside of cars, that target maximum speed should be low: “Traffic needs to flow at a neighborhood speed (15 mph [24 kph] or less is optimum) to make a human habitat that is safe and productive.” (Marohn, page 56)

In recent years there has been a substantial rise in pedestrian and cyclist fatalities, even as motorist fatalities have continued a long downward trend. The rising death toll among vulnerable road users was particularly noticeable during and following the pandemic. In Marohn’s words we find a good explanation:

“Most [traffic fatalities] happen at nonpeak times and in noncongested areas. … the traffic fatality rate is much higher during periods of low congestion. This is … because the transportation system is designed to be really dangerous, and traffic congestion, along with the slow speeds that result, is masking just how dangerous it is.” (Marohn, 117)

With many businesses closed and many people working from home, there was much less traffic congestion. And without congestion acting as a brake, people drove faster and more pedestrians were killed. That wasn’t intentional, but it was predictable – it was no accident.

* * *

As Jessie Singer explains, we find an extensive matrix of causes that contributes to “accidents” when we look beyond the individual making a mistake. That matrix very often includes racial and economic inequality, which is why poor people suffer more in nearly every accident category than rich people do.

Both racial and economic factors come into play in the current wave of pedestrian deaths. In the major city closest to me, Toronto, pedestrian deaths occur disproportionately among racialized, poor, and elderly people. These deaths also occur most often on wide arterial roads – stroads – in older suburbs.

Marohn’s words again are enlightening: “as auto-oriented suburbs age and decline … they are becoming home to an increasing number of poor families, including many who do not own automobiles.” (Marohn, page 43) When these residents need to walk across four, five or six lane high-speed arterial roads, the predictable result is pedestrian deaths among the most vulnerable. An obvious, though politically difficult, solution is to redesign these roads to bring speeds down to a safe level.

The inequality that contributes to “accidents” is buttressed in most North American cities by an elaborate legal framework telling people where they are allowed to live and work. That legal framework is zoning. In the next installment of this discussion we’ll look at the history and consequences of zoning.


Image at top of page is in public domain under Creative Commons CC0, from pxhere.