Hey, engineers: “Forgiving design and the forgiveness of slow speed are two very different approaches because they serve two very different needs.”


I’ll keep saying it until someone in the Down Low City Hall Bunker starts paying attention.


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 …