Forgiving design in action. Forgiving for drivers, not so forgiving for anyone else. (Source: Johnny Sanphillippo)

Forgiving design in action. Forgiving for drivers, not so forgiving for anyone else. (Source: Johnny Sanphillippo)

When something works in one situation, it often gets applied to many situations, and sometimes this choice is a big mistake. In the transportation world, it’s common to apply safety standards and designs from high-speed highways to city streets. Ironically, these “safety features” can be downright dangerous. Today, I’ll explain how we got here, why this is a problem, and what we can do about it.

The concept of forgiving design was developed by transportation engineers to lessen or avoid the impact of “run off the road” crashes. The general premise is that highways should have broad shoulders with gentle slopes and a roadside clear zone which is free of fixed objects such as light poles. These design elements provide drivers time to make corrective action if they start to drift off the road, and if they do crash, it is less severe.

Civil engineering students across the country learn these principles when studying about the geometric design of highways. They learn about the impact of shoulder widths, clear zones, slopes, and drainage features, as well as how to mitigate the dangers of fixed roadside objects through the use of barriers and crash attenuators. I have no doubt that lives have been saved on our high-speed highways as a result of these concepts, but they have been misapplied — resulting in negative unintended consequences — elsewhere.

I think at least part of the problem stems from the fact that minimal attention or training for engineers is given to the design of urban and local streets, resulting in a tendency to apply highway design principles to these streets in the name of safety. This backfires, because forgiving design features encourage speeding in places where people should be driving slow. When drivers see wide, straight stretches of pavement with no obstructions on either side, they intuitively think that it’s safe to drive fast. This is a big problem on streets (as opposed to highways or roads), because streets are places where people walk, bike, shop, live, work, etc. These activities are incompatible with and downright dangerous when mixed with high speed traffic. The bottom line is this: On streets, the design objective needs to flip from forgiving design to the forgiveness of slow speeds.

The concept of focusing on slow speeds for streets is something that is explored regularly here at Strong Towns, including my most recent article, which discussed how to create a system of safe, human-centered streets. The good news is that traffic engineers are increasingly talking about forgiving design vs. the forgiveness of slow speeds and Strong Towns is at the forefront with the #SlowtheCars movement. Attitudes are changing worldwide as we realize the power of slow.

The danger of mixing design standards was discussed in an article published several years ago in the Transportation Research Record (TRR):

Rather than attempting to function as freeways, the livable streets… instead address safety by discouraging the high-speed operating behavior that produces systematic error.  (Eric Dumbaugh, “Design of Safe Urban Roadsides,” TRR No 1961, pp 74-82)

The author goes on to state that European designers have long recognized that crashes at lower speeds are, by definition, more forgiving. This is why Vision Zero efforts in places like Sweden have been so effective. Meanwhile, safety efforts in the United States have focused more heavily on education and enforcement to the point of victim blaming. This has been less effective than focusing on design. The result is that the US is falling behind on traffic safety. See recent articles from the Economist or Curbed for more details. Education campaigns are important, but their effectiveness is limited unless they are accompanied with systematic design changes.

The following are some questions to help you decide if you should design your street with forgiving design or the forgiveness of slow speeds:

  • Do homes have direct access (via curb cut, driveway, etc) to the street?
  • Do businesses have direct access to the street?
  • Do schools, libraries, parks, senior centers, or other land uses which serve vulnerable populations have direct access to the street?
  • Will people be walking along the street?
  • Will people be walking across the street?

If you answered yes to any of these questions, you have a street, not a highway, and you should design for the forgiveness of slow speeds instead of applying forgiving design. The following table contrasts the difference:

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Here are a couple examples of forgiving design in action:

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This is I-80, west of the Salt Lake City International Airport. It has wide lanes, wide shoulders, a clear zone with a gentle slope, a wide median to separate opposing traffic and a cable barrier down the middle as an added precaution. If you’re going to let people propel themselves forward inside of high-tech machinery at speeds exceeding 70 mph (which we do daily), this is about as safe as it gets.

This is a parkway in Taylorsville, Utah. Vehicles travel at high speeds, but not nearly as fast as on the Interstate system. Notice the wide shoulders and the clear zone, with the power poles pushed as far away from the travel lanes as possible. As with I-80, this highway is designed for vehicles. People outside of vehicles are not safe here and this is pretty clearly understood by everyone. There is a sidewalk on one side of this street, but it’s rarely used. It has some physical separation from the travel lanes, which helps make it safer, but it could probably use either a barrier or additional separation to make it more comfortable for pedestrians and cyclists.

Here are some examples showing the forgiveness of slow speeds:

This is a residential street in Salt Lake City, Utah. This street was developed in the 1940’s and 50’s. This is the street where my grandparents built their first home. My dad and his siblings roamed freely and safely around this neighborhood as kids. This street has cars parked on it and a variety of fixed objects alongside it: poles, streetlamps, and trees. These all tell drivers to slow down and be alert. It is a classic, safe, charming residential street which has aged well. 

This is Main Street in Bountiful, Utah. The curbs have small radii, forcing drivers to slow down for tight curves. There are a wide variety of fixed objects right up against the street: light poles, planter boxes, trees, drinking fountains, etc. This is a safe place, built for people first. It also happens to accommodate cars. Not surprisingly, this street gets a lot of foot traffic.

This is State Street near downtown Salt Lake City. It's a street in transition. It has carried high levels of vehicular traffic for decades, but is increasingly being reworked to be comfortable and inviting for people on foot. It carries the same amount of traffic as before; those cars simply travel slower.

Forgiving design and the forgiveness of slow speed are two very different approaches because they serve two very different needs. When working on any road project, it is critical to clearly identify if you are dealing with a high speed highway or a low speed street.

Make that commitment, go all in, and avoid the messy, dangerous middle. The next time an engineer starts talking about clear zones and the need for wide shoulders as necessary safety improvements on a street you care about, call them out on it, and have a conversation about the forgiveness of slow speeds.


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About the author

Jon Larsen is a professional engineer in the state of Utah. He recently served as the chair of the board for the Congress for the New Urbanism (CNU) Utah Chapter. He has been a Strong Towns member since 2013.