Guest post by Jonah Finkelstein, EIT Spack Consulting
With the definitions discussed in Crash Analysis Definitions, we can now look into some of the tools provided by the Minnesota Department of Transportation (MnDOT) to help analyze crash rates around the state. This blog post will focus on MnDOT’s tool for intersection crashes known as the Intersections Green Sheets.
MnDOT supplies updated Green Sheets annually accounting for changes in crash data from the previous year’s logged crashes. Currently, the 2015 Intersection Green Sheet is the most up-to-date tool, with the updated 2016 Green Sheet expected to be released within the year. Using the link provided above, the Intersection Green Sheet can be downloaded for use.
The 2015 Intersection Green Sheet is an Excel file with the following three tabs:
- Statewide Averages – This tab shows the most up-to-date crash data based on roadway type and years of analysis. No modifications are required on this tab as it is only used for reference in the internal operations of the Intersection Green Sheet.
- Definitions – The Definitions tab provides the definitions and ranges of the intersection categories as well as the codes used in the Calculator tab. Again, there are no modifications required in this tab. It is only used to help the user understand the final Calculator tab.
- Calculator – The Calculator tab is where the crash information as well as the specific intersection characteristics are inserted. This tab is where the action is for a crash analysis.
With that sheet in hand, here’s the process we follow to complete a crash analysis for an intersection.
Log into to the Minnesota Crash Mapping Analysis Tool, selecting the County where the study intersection is located. After panning the view to the study intersection(s) in the map, set the boundary for the crash area. We typically use a circle centered on the intersection with a radius of 250 feet (hold control to select more than one intersection). Then, using the crash filter tool, and scrolling all the way to the bottom of the available filters, the years of analysis can be selected. The Intersection Green Sheets provide data for three, five, and ten years of crash data. Unless there is a specific reason at the intersection being analyzed, such as a new intersection layout or recent upgrades, a five-year crash analysis is recommended. Apply the filter to the selected crash radius and select the Reports tab. For the Intersection Green Sheets, the Crash Type Summary Report provides all the necessary information. Select this report and click Launch Report. A Crash Detail Report can also be printed to help review any crashes that are occurring near the intersection but may not be related to the traffic control device.
After reviewing the crashes, remove any crashes that are most likely not related to the intersection or its traffic control. For example, a run-off the road heading away from the intersection probably didn’t occur because of the intersection. A rear end crash 200 feet from the intersection might be related to a traffic signal or stop sign. Then, fill in the “Crash Data Input” section in the Calculator tab of the Intersection Green Sheets. This section compiles the number of study years, as well as the number of each crash severity type to help determine the corresponding rates. Finally, the “Section Code Input” section can be completed based on the intersection’s specific characteristics.
If turning movement counts were collected at the intersection, the daily entering volume and highest leg volume should be used from the counts. If not, MnDOT’s provided AADT Volumes can be used. When using these volumes, the highest leg volume will be the corresponding AADT value, while the total entering volume will be the summation of the AADTs on each leg of the intersection divided by two (since the AADT is both entering and exiting traffic). Then the traffic control device, intersection environment, and posted speed limit for the specific intersection can be input and…. VUALA, the Intersection Safety Screening report is updated based on the entered data.
The supplied report page is held within the Calculator tab and will show the number of crashes by each severity, the intersection characteristics, and the statewide crash rate comparison for both the total crashes as well as the fatal and serious crashes alone (K and A crashes). As mentioned in Crash Analysis – Definitions, the Critical Index is the most telling item produced. It is the ratio of the observed crash rate to the critical crash rate. A critical index of 1.00 or less indicates performance within expectations without deviation from statewide trends. Critical Indexes above 1.00 indicate that there is likely an existing safety concern at the intersection. Additional analysis and observation of the intersection should be complete to determine what is the cause of the high critical index and what mitigation can be completed to help increase safety.
Not in Minnesota? The MnDOT green sheets could still be used. Simply replace the statewide averages with your localized rates. The remaining calculations should all be valid and provide the insight needed for your study.