The Vision Zero Engineering Bucket
The engineering bucket consists of all the deaths that could feasibly be prevented through a change in the built environment (or, presumably, through AV technology). This is definitely the largest bucket, holding the largest potential for gain, which is why Sweden’s laser focus on re-engineering their streets has been so successful.
We can break the engineering category into two parts. We want to
- Reduce the number of conflicts / collisions, and
- Reduce the likelihood of a serious injury or death, given that a collision occurred.
The traditional engineering approach in the United States has been to focus almost exclusively on the first bullet, which is why we measure engineering countermeasures in terms of the number of crashes prevented. The wonderful addition from the Swedes is the second bullet - reducing the likelihood of serious injury, given that a collision already occurred. Focusing on this second bullet really means focusing on speed reduction.
The approaches to these two different goals are a bit different, as shown in the diagram below.
Strategy 1: Reduce the number of conflicts
Traffic engineering is already pretty decent at reducing the number of conflicts - there is extensive literature on the crash modification factor (CMF) of different treatments. Still, in the context of Vision Zero, there are probably two areas where we can improve:
First, we can look for ways to improve the deployment traditional engineering treatments. For example, given the current standards which dicatate how we evaluate locations for signals, can we make sure that every intersection that meets the criteria has a signal? Or, can we adjust the criteria to be more strict when it comes to safety, allowing us to deploy the traditional engineering countermeasures more often?
Second, much of the literature focuses on the more conventional engineering treatments: stop signs, traffic signals, etc. There a number of new treatments, many of them focused on interactions between vehicles and bicyclists or pedestrians, and as we continue to monitor their performance we will hopefully get more information about the relative effectiveness of each one.
Strategy 2: Reduce the likelihood of KSI
This is the secret sauce to the Swedish success, and is why Vision Zero is so obsessed with speed reduction.