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traffic engineering

Design-Oriented Traffic Safety Vs. Passive Safety

U.S. traffic safety has eroded compared to other countries. Focus on passive safety measures is partly to blame. (Graph from NYTimes)

While I was recuperating from my fall this winter, I spent a lot of time reading about international development and public health. “Poor Economics” by Abhijit V. Banerjee and Esther Duflo caught my attention because it makes an empathic case for understanding the cycles of disease and poverty, and explains how small, incremental changes can have huge benefits for vulnerable populations. On a case study of Ugandan school system corruption, for example,  embezzlement by district officials was reduced substantially by publishing school financial information in the papers. Once the information was public and officials were held accountable, corruption was reduced and schools received most of the money owed to them.

The example above implies that sometimes systems and institutions designed to provide public services fail, and when they do, work-arounds are needed. Banerjee and Duflo site the three “I”s as the primary problems facing well-meaning but failing institutions; ideology, ignorance, and inertia. While the authors use these concepts to explain international development studies in third world countries, they can also be used to explain many failing street design policies which have contributed to automobile domination in our public spaces, significant loss of life on our streets, and the entrenched auto-centric culture prevalent in many state and local DOTs.

I’ll explain why this is important. While places like the Netherlands have taken an active, grass roots stand against reducing road injuries and fatalities through safer road design, the U.S. has 32,000 traffic deaths a year (about a 60% higher per capita fatality rate than the Netherlands) and road design isn’t even remotely within our national public discourse. We tacitly accept these roadway injuries and fatalities as acceptable collateral damage for the privilege of owning and operating a personal vehicle in the most convenient way possible; excessively wide streets, insane roadway standards which encourage speeding and auto dominance, and alternative modes as mostly an afterthought.

Still, despite its head start and that cocoon of technology, the nation has steadily slipped behind other countries, becoming comparatively one of the most dangerous places to drive in the industrialized world. -Tanya Mohn, 2007, New York Times

Our auto culture has been dominated by passive safety measures. Bulkier cars, airbags, clear zones, wide streets; basically making our streets idiot-proof for motorists who want to drive as fast as possible. While total traffic deaths have decreased since 2005, and some passive safety devices have helped, pedestrian and cyclist deaths are on the rise. The core idea of passive traffic safety in the U.S. came from William Haddon, a medical doctor who teamed up with Senator Patrick Moynahan and Ralph Nadar in the late 1950s and 1960s to push legislation which protected drivers at all costs without influencing their behavior.

The orthodoxy of that time held that safety was about reducing accidents–educating drivers, training them, making them slow down. To Haddon, this approach made no sense. His goal was to reduce the injuries that accidents caused. In particular, he did not believe in safety measures that depended on changing the behavior of the driver, since he considered the driver unreliable, hard to educate, and prone to error. Haddon believed the best safety measures were passive.  – Malcolm Gladwell, The New Yorker, 2001

Passive safety was pushed at the expense of more holistic, design-oriented solutions which protect vulnerable road users and slow traffic. The problem isn’t mandating seatbelts and air bags; they save lives. The problem is focusing on passive safety and completely ignoring design. Driving behavior must be influenced for safer streets.  I still hear planners and engineers quoting Haddon, not fully understanding where the ideas came from or how the ideology has failed at creating livable streets.

So, getting back to the 3 “I”s, here’s how they apply to institutions which have influenced road design in the U.S. during the past 50 years:

  • Ideology: Traditional U.S. auto culture has pushed passive safety measures as king. Haddon, Moynihan and Nadar are mostly responsible for this 50 year old ideology. It needs to end.  Core ideas which have influenced road design and should now be questioned: “If the driver protected, that’s all that counts.” “If all traffic is going the same speed, no matter how fast, the road is safe.” “Clear, obstruction-less streets are best.” “Roads are designed for cars because almost everyone drives and that’s how Americans want to get around.” These ideas have been pervasive at the local, regional and national level and are outdated at best.
  • Ignorance: Many people who design streets may not actually spend time on them or understand how they work, or if they do, their only perspective is through a windshield. Experience is underrated and actually getting out into the neighborhood to see how streets are used (or would like to be used by communities) fills in a lot of knowledge gaps. On a personal level, streets are one of the few remaining public places in many neighborhoods, and top down, myopic engineering standards are often inadequate at dealing with the nuances of their use by communities.
  • Inertia: Most people in the U.S. as well as state and local DOTs have gotten so used to fast, wide, mono-use streets that anything which may interfere with free flowing traffic at maximum speeds  is deemed sacrilege. It’s just easier to keep the gears turning instead of having to fight with the powers-that-be about why you want to convert a traffic lane into a cycletrack.

While the U.S. threw up its arms and said, “We’re not going to change driving behavior so we might as well protect the driver”, the Netherlands saw road fatalities as unacceptable and that sensible road design was a way to make streets safer for everyone, even if it means de-prioritizing the automobile through a cultural shift.  The Netherlands instituted traffic calming, shared spaces, road diets, and other measures at a national level and saw results. The country refused to accept needless carnage on its streets.

While we can do the same in the U.S., it’s more of a challenge. The auto culture is more entrenched. The country more expansive and diverse. But we can start on the path of sensible road design by washing away the three “I”s mentioned above and fundamentally rethinking our roads using these 5 axioms:

  • Design influences behavior. Build streets more human-scaled, and drivers and pedestrians begin to act as equals. Build streets as highways, and automobiles dominate the space.
  • Speed is the enemy. This goes against almost everything traffic engineers learn in school, but agencies have to stop being afraid to inconvenience motorists. I don’t care if it’s a major arterial road that an important politician drives on everyday. Traffic speeds over 35mph are more likely to kill drivers, pedestrians and cyclists, and every single element of street design needs to communicate to drivers that speed is not acceptable.
  • Material collateral damage is acceptable.  Trees, curbs, narrower streets, unique intersection angles, medians and the like which slow driver speeds at the risk of low-speed collisions with a fixed object are OK.  Vertical elements are good. Remember the first axiom: If drivers sense they no longer dominate the road, they will slow down and be more attentive to their surroundings. I’d rather have a driver hit a tree at 15mph than careen into a sidewalk at 35mph.
  • Human collateral damage is unacceptable. This should go without saying, but given the fact that we design many of our streets like highways, sometimes I think U.S. auto culture values speed and convenience over human life.
  • A street is a place first, a conduit second. As Mikael Colville-Andersen mentioned, we’ve gotten into the habit of engineering something that is, by nature, very organic and personal. Think of the street where you grew up. Do you describe it using level-of-service measurements? Vehicle miles traveled per day? Traffic delay? When you walked down the street to ask your friend to come out and play when you were 8, none of this mattered. It still doesn’t. We just pretend it does.

Not only do we get safer cities with design-oriented approaches, we get more livable streets, more transportation equity, and nicer neighborhoods. While certain cities “get” this approach and progressive agencies are doing great work, the national culture is dominated by acquiescence concerning street design – an issue that is at least partially responsible for 35,000 fatalities a year. The dialog needs to change from, “Sometimes auto accidents just happen” to “This is why accidents happen and here is how we change our streets to prevent them.”


Additional Reading:
Journal of Injury Prevention: Safety in numbers: more walkers and bicyclists, safer walking and bicycling, 2003.
The New Yorker: Wrong Turn: How the fight to make America’s highways safer went off course., 2001
London Cyclist Magazine: Dutch Campaigners explain why the Netherlands is now so bike friendly


Bicycle Culture by Design: Mikael Colville-Andersen

Mikael Colville-Anderson, urban mobility expert with Copenhagenize Consulting.

Engineers are brilliant problem solvers. They just need to be told which problems to solve. They are rarely leaders. They are the Can Do team. If we design a city properly, they will make it work. But as it is now, we are living in The Matrix, because traffic engineering goes unchecked and uncriticised. The 85th Percentile, for example, is a joke. An archaic study that doesn’t work. And yet it’s the first thing you learn when studying engineering. Time to change things.

On Being a City Planner In a Room Full of Engineers

A few words of encouragement if you’ve ever been the only non-traffic engineer in a room full of traffic engineers.

  1. It’s OK to question Level of Service and traffic volume projections. They’ve often been wrong before. They will be wrong again.
  2. It’s OK to advocate for narrower lanes.
  3. It’s OK to use the phrases “fast”, “anti-urban” and “does not meet livability goals” when describing one way couplets.
  4. Protected bike lanes are no longer radical ideas, even if they mean taking traffic lanes away from automobiles.
  5. Your intuition is correct. Sharrows on high volume streets are dangerous and should not be used just to placate cyclists.
  6. Full time on-street parking is not an impediment to traffic flow, even on urban arterials. It’s a retail-booster and a revenue generating traffic calming device.
  7. It’s OK to talk about big picture things when the conversation focuses on minutia.
  8. It’s OK to expect something exceptional and transformational from a project.
  9. It’s OK to suggest that the project engineers actually walk or bike on the street they are designing.
  10. It’s OK to question neighborhood design speeds in excess of 20mph, the 85% percentile rule, intersection geometrics and clear zones, even if you’re not an engineer.
  11. Aesthetics are just as important as function. Signal poles, bus stops, sidewalks, and the entire streetscape are as much a part of urban design as buildings and parks.

…learning how to make cities rich and fecund and great places to be so we’re comfortable and healthy and happy is the biggest problem we face. The only way we’ll not go crazy is to build beautiful, rich, life-enhancing cities….The majority of open spaces in cities are streets. That means the street system is too important to leave solely to transportation engineers. They’re way too important to leave to just moving traffic. So I’m interested in cities because they are the design problem for a habitable planet. – Laurie Olin

Keep on going.


Friday Reading

The Tissue of Straight Lines: a meditation on NYC’s grid street network from Kneeling Bus, one of the few blogs I read which leaves me saying, “I wish this guy wrote more articles.”

The grid that originated in 1811 will never be finished as long as it remains in place, because it will never stop challenging its inhabitants to infuse its neutral, rectangular blocks with the vibrant content of humanity and culture.

Designs For Working: Malcolm Gladwell on Jane Jacobs and how her ideas have been co-opted by corporations looking for collaborative, social working environments. See: Zappos’ plan for Las Vegas.


Sparely populated suburbs may look appealing, she said, but without an active sidewalk life, without the frequent, serendipitous interactions of many different people, “there is no public acquaintanceship, no foundation of public trust, no cross-connections with the necessary people–and no practice or ease in applying the most ordinary techniques of city public life at lowly levels.”

“The Solomon Curve, developed in 1964, states that those driving slowest will be at the greatest risk of crashing. This outdated model, which ignores pedestrian safety entirely, still guides traffic engineering toward higher speeds.” StreetsBlog

This has been going around like wild fire. The 85th Percentile Rule in Traffic Engineering from Copenhagenize: an outdated way of setting traffic speeds which ignores neighborhood context, pedestrians and cyclists. This model is still being used in almost every city in the world.

Imagine a street where the average speed is 50 km/h. If the speed limit is reduced by 5 km/h then, according to this archaic model, the drivers are allegedly exposed to a higher risk. What is most shocking is that this entire concept completely ignores pedestrians and cyclists. Another horrific conclusion from this graph is that when you increase the speed limit, the crash risk is alleged to be less than for slow speeds.

The Institute of Traffic Engineers wrote: “The 85th Percentile is how drivers vote with their feet”. They forgot to mention that, when it comes to establishing speed limits in cities, pedestrians and cyclists are excluded from this election. They don’t even get the chance to go to the polls.

All this right now in 2012. In your street. With your tax money.

And finally, the benefits of preserving local manufacturing districts. Proximity is Creativity: Unlocking the Value of the Garment District from Urban Omnibus. There are some things China just can’t do.

Let’s say it’s your last year at school, and you have a set of starter designs that are very marketable. What happens next? You need to get someone to make your production patterns; you need to able to source fabrics; you need to be able to sell, to have access to the stores. So let’s say you come up with a 20-piece order. You can go out into the Garment District, find a cutting room or a sewing room, and have your 20 pieces produced and shipped to a store. You can’t get only 20 pieces made in China, not today, not ever. That is what validates what goes on in the District today: the capacity to produce short runs, samples made with a quick turnaround time.