Crash Analysis: Corridor Crash Rates

Guest post by Jonah Finkelstein, EIT Spack Consulting

With the general crash definitions discussed in Crash Analysis Definitions, and the process and tools used to conduct an Intersection analysis reviewed in Crash Analysis: Intersection Crash Rates, we can now look into Minnesota Department of Transportation’s (MnDOT’s) tool for corridor crashes known as the Section Green Sheets.

As discussed in Crash Analysis: Intersection Crash Rates MnDOT supplies updated Green Sheets annually accounting for changes in crash data from the previous year’s logged crashes. The Corridor Green Sheets are updated at the same time as the Intersection Green sheets and contain the same three tabs:

1. 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.

2. 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.

3. Manual 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.

The main difference in utilizing the Section Green Sheet compared to the Intersection Green Sheet is that a corridor analysis will need a few more inputs. In order to assure proper analysis, various roadway characteristics including daily traffic volume, number of lanes, and median type are needed. will vary throughout a corridor’s length. Here are the basic ones you’ll need:

1. Environment – Generally refers to the type of surrounding development; Urban, Suburban, or Rural.
2. Design – For this purpose, the splits are Freeway (think Interstate), Expressway (high speed with some access control and primary intersection control by signals), or Conventional.
3. Number of Lanes – The sheet can accommodate one to eight lanes.
4. Median Type – Broken into four categories here; Not Divided/No Median, Barrier (includes concrete, guardrail, and cable), Curb, or Divided/Depressed Median.
5. Average Daily Traffic (ADT) – Either use volumes you have counted or an agency’s provided volumes, like .

Depending upon the length of the corridor being analyzed, these characteristics may all be the same or have a few shifts between different locations. For example, let’s assume a sample roadway section with three major intersections along the study corridor. With a simple labeling (A at the start, E at the end, and B, C, D for the major intersections), a breakdown of these compiled characteristics into segments along the corridor might look like this:

With the characteristics determined, measure the length of each segment to finalize the list of roadway inputs for use in the Section Green Sheets.

The Crash Type Summary Report for each segment can then be generated from the Minnesota Crash Mapping Analysis Tool (MnCMAT) by using the polygon tool (to create a shape that encompass each segment length) or using the segment selector (holding ‘control’ down for multiple selections). At this point, you must decide whether to include intersection crashes in the section analysis or not. Check with the governing agency, who may have reasons to exclude intersection crashes in this analysis. If you are completing separate intersection crash analysis, you may also want to exclude intersection crashes. Using the MnCMAT tool, filters can be used to focus on the crashes desired.

We can now begin filling out the Corridor Green Sheet. Start by filling in the “Crash Data Input” in the Manual Calculator tab. This section is the same as the Intersection Green Sheet and compiles the number of study years, as well as the number of each crash severity type to help determine the corresponding rates. Make sure that the ”Junction Included” tab is toggled to the correct field depending on how crash data was collected. Using the earlier defined segments, fill in the “Section Code Input” section with the characteristics determined in the segment analysis.

Much like the Intersection Green Sheet Summary, the supplied report page is held within the Manual Calculator tab and will show the number of crashes by each severity, the section characteristics, and the statewide crash rate comparison for both the total crashes as well as the fatal and serious injury crashes alone (K and A crashes). As mentioned in the earlier 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 along the corridor. 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.