Basic Analysis Procedures

The following sequence of steps should be followed to estimate the safety performance of an interchange using ISAT. Most steps will be performed each time a safety assessment is made of an interchange area. Other steps will require user input primarily on an annual basis. The general analysis procedures are as follows:

Step 1—Start Microsoft Excel spreadsheet software.
Step 2—Start ISAT.
Step 3—Review default data and update values to reflect current local conditions.
Step 4—Identify individual components of analysis area.
Step 5—Enter general interchange data.
Step 6—Enter interchange element data.
        Mainline freeway segments
        Ramps
        Crossroad ramp terminals and intersections
        Crossroad roadway segments
Step 7—Perform calculations.
Step 8—Review results.

Details on each of these steps are presented below.

Step 1-Start Microsoft Excel Spreadsheet Software

As stated in the Excel Basics section of this manual, Microsoft Excel spreadsheet software can be started from the Start button by clicking on the Programs option, and then on Microsoft Excel. There may also be an icon on the desktop or on a toolbar that can be clicked to start Excel.

Step 2—Start ISAT

ISAT can be started from within Excel by clicking on the File menu, clicking on the Open option, browsing to find the ISAT file location, and clicking on the ISAT file name. It is also possible to use Windows Explorer from the computer desktop to find the ISAT file location. Clicking on the ISAT file name will automatically open Excel spreadsheet software (if it is not already open) and start ISAT. Thus, with Windows Explorer the user can bypass Step 1 and go directly to Step 2.

It is also possible to set up an ISAT icon (shortcut) on the computer desktop to start ISAT without directly opening Excel spreadsheet software first. A shortcut of this type can be created from the Programs option in the Start menu or from Windows Explorer. It can then be moved to the computer desktop.

Step 3—Review Default Data and Update Values to Reflect Current Local Conditions

After starting the ISAT application, the worksheet labeled Input-General appears. This is the main control worksheet for ISAT and guides the user through the data entry steps and then to the data processing procedure. Initially, when no data have been entered, only the input worksheets are accessible to the user at the bottom of the screen. However, the user also has access to the default input worksheets through the active hyperlinks found on this worksheet. All subsequent hyperlinks labeled "Home" return you to this page.

Users will be able to proceed with analyses using the default data provided with the program or may substitute data obtained by their own agency that they consider more appropriate than the original default values supplied with the program. There are three types of default data:

• Safety Performance Functions (SPFs): An SPF is a statistical model used to predict the safety performance of specific sites based on their characteristics. An SPF is thus represented by an equation that describes the functional relationship between predicted crash frequencies and site characteristics. Default SPFs are provided for total (TOT) and fatal and injury (FI) crash severities for subtypes of each interchange element (i.e., mainline freeway segments, ramps [and acceleration lanes], crossroad ramp terminals and intersections, and crossroad roadway segments), where subtypes differ by geometrics or functional characteristics. The default values supplied with ISAT were developed through previous and ongoing safety research. In most cases, it is anticipated that ISAT users will not modify the default values associated with the SPFs, but ISAT does not prohibit the user from doing so. It is recommended that these values be changed only when an agency has better and more recent research results available, but it is also recommended that the user routinely check or verify the default values associated with default SPFs to ensure the values have not been changed prior to performing calculations. Alternatively, if the default values have been modified at some point, the user should reverify that the values are still correct.
• Calibration Coefficients: Calibration coefficients provide the capability to adjust the calculations performed within ISAT to better reflect local safety experience. The user should review the calibration coefficients to make sure they seem logical. If the values seem unreasonable, the user should modify the values as appropriate, which may require performing the calibration procedures as described in Calibration Coefficients section of this manual.
• Crash Distributions: The default crash distributions include the severity and crash type proportions for specific types of interchange elements. The user should review these distributions to make sure they appear consistent with local experience. The user should modify these values as appropriate (see the Crash Distributions by Severity and Type section).

The default input worksheets for the SPFs, calibration coefficients, and crash distributions may be accessed by using the appropriate hyperlink on the Input-General worksheet. To edit default values, simply click in the desired cell, delete the old value, and retype the new; note, however, that other cells on these worksheets are protected and cannot be modified. Before the user changes any of the default values, it is recommended that the file be saved under a different name to avoid the possibility of permanently losing data that might be desirous for a future analysis. The buttons provided to clear input or output data will NOT return SPFs, calibration coefficients, or crash distributions to the original default values.

Step 4—Identify individual Components of Analysis Area

The user will perform this step outside of the program, but it is a recommended step for organizational purposes. The user should develop a sketch of the analysis area illustrating, and numbering, each of the individual components to be considered in a given analysis. A sketch similar to figure 1 will help the user visualize the various components included in the analysis. The sketch will help the user during the data input process, primarily when inputting data for the individual components, and will help reduce the likelihood of errors during data entry. Further guidance on how to subdivide an interchange area into individual components is provided in the Input Requirements, Default Data, and Applications of ISAT sections of this manual.

Step 5—Enter General Interchange Data

The user enters general information concerning the analysis on the Input-General worksheet. On this input worksheet, the user provides general information describing the analysis and whether crash data will be incorporated into the computations. The General Interchange Inputs section provides more details concerning the general input data. The user can return to the Input-General worksheet by using the Home hyperlink located in the upper left corner on all worksheets, except those associated with the output reports.

Step 6—Enter Interchange Element Data

The user enters data for the individual components of the analysis area on the respective worksheets:

• Input Mainline Freeways.
• Input Ramps.
• Input Ramp Terminals.
• Input Crossroad Segments.

After all of the input data have been entered, the user should click the Home hyperlink at the upper left corner of the respective input worksheet to return to the Input-General worksheet.

Step 7—Perform Calculations

Having entered all of the required input data, the Perform Calculations button on the Input-General worksheet can now be used to begin the processing calculations and generate output. Clicking this button begins the sequence of actions that performs the calculations and generates output.

Step 8—Review Results

When the user clicks the Perform Calculations button on the Input-General worksheet, ISAT automatically takes the user to the first output report worksheet (i.e., Output-General). On the Output-General worksheet, output results are summarized in the following manner:

• Number of predicted crashes for entire interchange area.
• Number of predicted crashes by interchange element type.
• Number of predicted crashes by year.
• Number of predicted crashes by collision type.

On separate output worksheets for mainline freeway segments, ramps, crossroad ramp terminals and intersections, and crossroad roadway segments, the numbers of predicted crashes by collision type and severity level are summed across the individual components of the respective interchange elements. In a separate table results are provided for individual components of the respective interchange elements. Each of the output worksheets can be viewed by clicking on the appropriate tab found at the bottom of the workbook.

FHWA-HRT-07-045

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