A Crash Tree Diagram is a simple data visualization tool designed to help identify crash and roadway characteristics for potential safety improvement. To learn more about what a Crash Tree Diagram is, see the What is a Crash Tree Diagram article.
Steps to create a Crash Tree Diagram
From the Application Dropdown Menu in the top left corner of the screen, click Crash Tree Diagrams.
With the Crash Tree Diagrams page open, begin by defining the data you would like to include in your Crash Tree Diagram. This can be done by modifying the Date Extents, and by adding Primary Filters.
The Date Extents define the range of data to include in your Crash Tree Diagram. Click on the Date Extents located in the top right corner of the screen, and select the desired date range from the menu.
The Primary Filters allow you to determine what crash data will be displayed in your Crash Tree Diagram. For example, you can define a geographic area (County, city, district, etc.), or a type of crash (Fatal, and Serious crashes). Only crashes that meet the Primary Filter will be displayed in the Crash Tree Diagram.
In the example below, I have selected District 3 as one of my Primary Filters. This means my diagram will only contain crashes that occurred in District 3.
With your Crash Tree Diagram data extents defined, you can begin building your Crash Tree Diagram by adding Digram Filters to the Filter Bar at the top of the screen, or by selecting one of the recommended filters below the Tree Diagram. Each Secondary Filter will represent a new row in your diagram. With your first filter selected, the Crash Tree Diagram will begin to populate on the screen. In the example below, I have selected Urban/Rural = Urbanized Area as my first Diagram Filter.
You will note that Urbanized Area appears to the left of the Diagram on the first row. The percentage of crashes that occur in Urbanized Areas appears at the top of the cell, and the total number of crashes appears below.
Clicking the Plus icon in the field to the right will display the distribution of all crashes among the other data points. In this example, all other districts in the state. Note: when more than 25 unique values exist for a selected filter, the not filtered values will be consolidated into a single “≠” value. For example, if I select City = Nashville as my filter, then the non filtered values will display to the right as City ≠ Nashville, rather than listing all cities that are not Nashville.
I can continue to build out my Crash Tree Diagram by continuing to add filters. As a general practice, you want to select the fields with the highest value as the filter when building a Crash Tree Diagram. For example, when adding my next filter, Intersection Related, I will select True from the dropdown, as it has the highest number of crashes, and will help us identify high-crash locations, and crash types.
However, there may be times when you don’t want to select the value with the most crashes. In this case, when looking at Intersection Related crashes, I do not want to explore Rear End collisions, as they are generally property damage only crashes, and are to be expected at a signal controlled intersection. Instead, I may select Angle (Other) or Left Angle crashes, as they are crash types that can be mitigated by countermeasures. This is where you will use your engineering judgment to investigate your crash data.
When your Crash Tree Diagram is complete, you can export the diagram by clicking the Download icon in the top right corner of the screen and selecting Image. This will download an image file of the Diagram to your computer.
Note: the diagram will download exactly as it is displayed in the application. So you may want to click the + icon or the - icon to hide or show fields in the diagram as desired.