U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-08-050
Date: May 2008
Surrogate Safety Assessment Model (SSAM)–SOFTWARE USER MANUAL
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This document is a software user's manual for the Surrogate Safety Assessment Model (SSAM). It provides guidance on the installation and use of the software, which is used to perform surrogate safety analysis of a traffic facility, such as a signalized intersection. It does so by processing vehicle trajectory data from a given microscopic traffic simulation to identify the frequency of conflicts-where two vehicles are on a collision course and one vehicle must make an evasive maneuver to avoid a collision. Such surrogate measures have been shown to correlate to some degree with the historical frequency of crashes at intersections. This technique may be of use to researchers, transportation engineers, construction and design companies, and safety engineers.
Director, Office of Safety
Research and Development
Technical Report Documentation Page
Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
LIST OF FIGURES
The Surrogate Safety Assessment Model (SSAM) is a software application designed to perform statistical analysis of vehicle trajectory data output from microscopic traffic simulation models. The software computes a number of surrogate measures of safety for each conflict that is identified in the trajectory data and then computes summaries (mean, max, etc.) of each surrogate measure. This document explains the installation, configuration, and usage of the SSAM software, focusing primarily on the graphical user interface features. This chapter summarizes the organization of the document.
This document explains terms and definitions pertinent to the installation and operation of the SSAM software. The major chapters of the user manual are outlined as follows and are in chronological order.
This section describes how to install the SSAM software using the automated SSAM installation program, SSAMinstall.exe. The section also describes how to uninstall the SSAM software.
This section describes how to use the graphical user interface of SSAM to perform conflict analysis on simulation trajectory data files. The Analysis section walks through an example to demonstrate the configuration and the usage of the available views and filters to analyze the simulation data. The Usability section reviews the Help menu, the different viewing options available in the SSAM software, and the functionality provided by the tool to export data from a SSAM case document to a .csv file, which can be viewed in Microsoft® Excel. The Error Checking section provides a description of some of the error checking performed on user input.
This section explains the terms and measures necessary to interpret the data viewed on the SSAM software screens.
Figure 1. Screen Capture. Installation Program, SSAMinstall.exe.
Double-click SSAMinstall.exe to bring up the InstallShield™ Wizard screen as seen in figure 2. At this point, the installer initializes itself and looks for a Java Virtual Machine (JVM) on the local machine. If a JVM is not found on the local machine, the installer will install the appropriate JVM. The SSAM software has been developed and tested for Windows XP; however, it was built with Java technology and other platforms may be supported in the future.
Figure 2. Screen Capture. InstallShield Wizard Screen.
Figure 3 . Screen Capture. Installer Screen-InstallShield Welcome Screen.
Figure 4. Screen Capture. Installer Screen-Directory Name.
As seen in figure 4, the installer will install the SSAM.exe file in the C:\ProgramsFiles\FHWA\SSAM directory by default. The user can choose to install the SSAM software in a different path by clicking Browse. Click Next to bring up the window seen in figure 5.
Figure 5. Screen Capture. Installer Screen-Option Choices Typical or Custom.
As seen in figure 5, the installer gives the user the option of either a typical or a custom install. In a typical install, the user does not have the option of selecting software features to be installed. In the case of SSAM, all the functionality is bundled into a single feature. As a result, choosing the custom installation is not necessary. Figure 6 shows the screen when the user selects the typical install option and figure 7 shows the screen when the user selects the custom install option.
Figure 7. Screen Capture. Installer Screen-Custom Installation Option.
Click Next to proceed with the installation of the SSAM software. The installer will install the SSAM software on the local machine. Figure 8 shows the screen displayed as the installation process is being carried out.
Figure 8. Screen Capture. Installer Screen-Installation Process.
Figure 9 shows the screen displayed when the installation is successful.
Figure 9. Screen Capture. Installer Screen-Installation Complete.
Click Finish to complete the installation of the SSAM software. A shortcut to the SSAM.exe file is created in the Programs menu. Figure 10 shows where the SSAM launcher and the SSAM uninstaller are installed. There is no need for the user to reboot the computer to begin using SSAM.
Figure 11. Screen Capture. Uninstaller Screen-InstallShield Welcome Screen.
Figure 12. Screen Capture. Uninstaller Screen-Product Uninstallation Features.
As mentioned previously, all the functionality of the SSAM software is bundled in one feature. Uninstalling SSAM implies that the Full SSAM application must be uninstalled. Figure 12 shows that the uninstaller will remove this installed feature from the local machine. Click Next to bring up the screen shown in figure 13.
Figure 13. Screen Capture. Uninstaller Screen-Uninstalling from Local Machine.
This screen informs the user that the SSAM software is about to be uninstalled from the local machine. Click Next to instantaneously uninstall the SSAM software from the machine and bring up a screen that tells the user that the uninstall process is complete. This screen is shown in figure 14.
Figure 14. Screen Capture. Uninstaller Screen-Uninstall Process Complete.
Click Finish to complete the process. None of the SSAM options will appear in the Programs menu after the uninstall process is complete.
Launching the SSAM software
Figure 15. Screen Capture. SSAM-Beginning Screen.
CHAPTER 2. Graphical user interface
This section of the user manual is divided into the following subsections:
This subsection provides a general description of the menus, menu items, toolbars, and screens available in SSAM.
This subsection describes how to use the configuration screen so that SSAM can process the required data files to extract information based on user-defined parameters.
This subsection gives an overview of the conflict and summary screens. It also describes how the filter provided by SSAM can be used to view all or some of the processed data in the conflict and summary screens.
This subsection explains how multiple files can be saved and viewed using the menus and menu items provided by SSAM. This section also describes how the software can be used to export data from a case document to a .csv file viewable in Microsoft® Excel.
Figure 16. Screen Capture. SSAM Screen-Beginning Screen with Callouts.
As seen in figure 16, the graphical user interface can be divided into the following four main parts:
1. Case document viewer
This part of the screen is used to view the data related to the selected case document or trajectory file.
A case document is selected by double-clicking on the case document in the case document viewer.
This part of the tool is used to save, open, view, and export case documents. It also contains the Help menu.
3. The toolbar area
This part contains buttons to save, open, view, and export case documents.
4. The internal frame
This part contains six tabs. These tabs will be viewable in the internal frame when a case document is created. The available tabs are summarized as follows:
Figure 17 labels the buttons available in the toolbar section.
Figure 17. Screen Capture. SSAM Screen-Toolbar Section Buttons.
As seen in figure 17, the toolbar section of the tool includes the following buttons:
Figure 18 and figure 19 show options available through the File menu and Window menu, respectively.
Figure 19. Screen Capture. SSAM Screen-Window Menu.
If SSAM was used previously to perform analysis and the data was saved, a list of the saved case documents will display in the case document viewer.
Creating a New Case Document
This section walks through the steps of creating a new SSAM case document. This presentation assumes that SSAM has been started for the first time and does not contain a list of case documents in the case document viewer. To create a new SSAM case document, point to New in the File menu or click New in the toolbar. This will bring up the file selection window where the user can type in the desired file/case name. Figure 20 shows the user interface after creating a new case document.
Figure 20. Screen Capture. SSAM Screen-Configuration Tab.
The case document 7692 yale road & hocking avenue_ssam appears in the case document viewer. The Configuration tab is selected by default in the internal frame.
As seen in figure 20, the Configuration tab displays the following information:
Figure 21. Screen capture. SSAM Screen-Configuration Tab Displaying Case Information.
Click Edit Info to change the case information. This will allow the text areas in the case file section to be edited and activate the apply change button. For example, figure 21 displays the following information:
To apply the change to the case information section, click Apply Change. This activates the Edit button and disables the text boxes as shown in figure 20. Note that this is not the same as saving the case document. The Apply Change button simply applies the changes to the Configuration tab of the internal frame so that case information exists on the screen even if the user switches tabs.
Adding Trajectory Files to a Case Document
Users can also select trajectory files that need to be analyzed. Click Edit to enable the case files in the threshold section of the Configuration tab. Figure 22 shows these fields activated.
Figure 22. Screen Capture. SSAM Screen-Configuration Tab Displaying Case Files.
Clicking Edit enables the user to click Apply while the Edit and Analyze buttons are disabled. To add a trajectory file to the case document, click Add. This will bring up a file selection menu through which single or multiple files can be selected.
Figure 23. Screen Capture. SSAM Screen-Configuration Tab Displaying Case Files: File Selection.
Figure 24. Screen Capture. SSAM Screen-Configuration Tab Displaying Case Files: Files Added.
Figure 24 shows five files added to the case files section. Note that the files selected have a .trj extension. In this example, default maximum TTC and maximum PET values in the Conflict Thresholds section are not changed.
Once the user has selected the case documents, changes can be applied to the case document. The user can click Edit again to add or delete any of the trajectory files in the case files section. Just as multiple files can be added, multiple files can be selected and deleted by clicking Delete in the case files section. Click Apply to activate the Edit and Analyze buttons as shown in figure 25.
Processing Trajectory Files
Click Analyze to start the analysis process. The SSAM software starts processing the data from each trajectory file to extract conflict data information from these files. The processing of trajectory files can take a long time depending on the size of each file and the number of trajectory files that need to be analyzed. The SSAM software presents the user with progress information about the processing of the trajectory files as shown in figure 26.
Figure 26. Screen Capture. SSAM Screen-Configuration Tab Displaying Case Files: Analysis Progress.
Clicking Analyze displays a window which shows progress information. In figure 26, 7 percent of the first file is complete and 1 percent of the total processing is done. This screen prevents the user from selecting any other options except Cancel. The screen provides the user with an option of canceling the entire analysis request. After the processing is complete, a message informs the user that the analysis process is finished without any errors. This can be seen in figure 27.
Figure 27. Screen Capture. SSAM Screen-Configuration Tab Displaying Case Files: Analysis Complete.
This message means that relevant conflict data can now be viewed in the Conflict tab of the internal frame. Internally, once the processing of the files is finished, the tool immediately starts calculating summaries, which can be viewed in the Summary tab of the internal frame. The software then informs the user that the summaries have been calculated, as seen in figure 28.
Figure 28. Screen Capture. SSAM Screen-Configuration Tab Displaying Case Files: Summary Complete.
The user is now ready to analyze the conflict and summary data obtained from the trajectory files. The next section of this manual covers the Conflict, Summary, and Filter tabs of the internal frame which help the user analyze the output conflict data populated by the SSAM software.
The analysis portion of the SSAM software allows users to filter conflicts, graphically visualize the conflicts within VISSIM .inp files, export maps, and perform t-tests between alternative case studies.
Note on Units of Analysis
SSAM does not allow user selection of the units of display and will perform no conversions (e.g., from metric to English or vice-versa). All values are interpreted and displayed in the units specified in the .trj files processed by SSAM. Some simulation vendors allow users to specify the units used in their simulation as metric or English; however, note that the units specified in the simulation may or may not affect the trajectory file output.
Using the Conflict Tab
Figure 29 shows the Conflict tab after the tool is done analyzing the trajectory files. Notice that the first row of the conflict tab displays, "NO FILTER APPLIED." This means that the data shown is a complete result set obtained from processing trajectory files.
Figure 29. Screen Capture. SSAM Screen-Conflict Tab.
The Conflict tab displays a table in which each row represents a conflict and each column represents certain information about that conflict. There are a total of 40 columns. As seen in figure 29, the Conflict tab provides a scrollable view of the conflict data. The size of each column can be adjusted. View the Terms and Definitions section of this document for a description of each column in the conflict table. Users can sort data by column in the Conflict tab by clicking the column header.
Using the Summary Tab
Figure 30. Screen capture. SSAM Screen-Summary Tab.
The data shown is a summary of the unfiltered conflict data in the Conflict tab. The Summary tab contains two tables. The first table provides a statistical summary of conflict data magnitudes, while the second table provides a summary of the total number of conflicts and the total number of conflict for each type seen in the Conflict tab. The first table provides statistical data on the following SSAM measures, which are defined in the appendix:
The statistical data is provided in terms of the minimum, maximum, mean, and variance of each of the above stated SSAM measures.
The second table provides a summary of the total number of conflicts broken down by type of conflicts. The four types of conflicts are
The logic to classify conflicts into these types is summarized as follows:
A conflict angle is calculated for each pair of conflicting vehicles, based on the angle at which these vehicles converge to a hypothetical collision point. See the definition of clock angle in the appendix for more detailed definition. Very small (acute) angles indicate that the vehicles were on nearly the same trajectory (e.g., a rear-end event might have a conflict angle of 0 °), whereas large angles (plus or minus 180 ° being the largest) indicate that the two vehicle were on a head-on course. An angle of plus or minus 90 ° indicates two vehicles on perpendicular paths. The angle is expressed from the perspective of the first car to arrive at the collision point. Positive angles indicate the second vehicle is approaching from the right, and negative angles indicate the second vehicle is approaching from the left. Thus, the range of angles is from negative 180 ° to positive 180 °.
Conflicts classification is generally based on the conflict angle, although link and lane information may affect the classification. For example, if two vehicles have a conflict on the same link and lane, then the conflict is a rear-end event regardless of the conflict angle. If both vehicles are on the same link and one of the vehicles changes lanes, then it is a lane-change event regardless of the conflict angle. However, link and lane information is not used if (a) it is not provided in the trajectory file, (b) the vehicles are on differing links, or (c) either vehicle changed links over the course of the conflict event. In such cases, the conflict angle is used for classification as follows:
The threshold angles (30 ° and 85 °) were determined by limited experimentation, though there are cases where classification is difficult. One such case involves a vehicle traveling in a traffic circle (within a roundabout) which crashes/conflicts with a vehicle entering the roundabout. Determining the appropriate type can be subjective if only one type can be selected. It is possible that these angle-thresholds will be exposed for user configuration in a future version of SSAM.
Notice that in figure 30 both tables provide a combined summary for all trajectory files as well as a break down of summaries by each trajectory file. These summaries are circled in figure 30. Unlike in the Conflict tab, data in the Summary tab cannot be sorted. The columns in the Summary tab can be resized and positioned differently.
Using the Filter Tab
Figure 31 shows the Filter tab of the internal frame with default settings. Large amounts of data in the Conflict tab can make finding conflicts which have certain commonalities or follow a certain pattern difficult for the user. The Filter tab allows the user to filter the data in the Conflict tab using the five criteria shown in figure 31.
Figure 31. Screen Capture. SSAM Screen-Filter Tab with Default Settings.
Figure 32. Screen capture. SSAM Screen-Filter Tab with Filter Settings.
Figure 32 displays the filter settings shown when the user chooses to view all the conflicts with
Applying the Filter
Figure 33. Screen Capture. SSAM Screen-Filter Tab with Filters Applied: Confirmation Message.
The Restore Defaults button restores the filter settings to all the default settings. It also restores the content in the Conflict tab and the Summary tab to show all conflicts and summaries for those conflicts. Figure 34 shows the Conflict tab after the filter is applied. Notice that the first line in the window now displays, "FILTER APPLIED."
Figure 34. Screen Capture. SSAM Screen-Conflicts Tab: Filtered Conflicts Table.
Figure 35. Screen Capture. SSAM Screen-Summary Tab: Filtered Summary Tables.
The summary tables now show the following kinds of summaries in chronological order:
Using the T-Test Tab
Figure 36. Screen Capture. SSAM Screen-Ttest Tab.
There are three sections in the ttest tab:
Adding Another Case Document
Users can select additional case documents to be compared with the current case document. Click the Edit to enable the SSAM files and t-test configuration sections of the t-test tab as shown in figure 37. Notice once Edit has been clicked, the Apply button is activated and the Edit, Analyze, and Export buttons are disabled.
Figure 37. Screen Capture. SSAM Screen-Ttest Tab SSAM Files.
To add a new case document file, click Compare opened file with in the SSAM files section. This will bring up a file selection menu through which single files can be selected. Figure 38 shows the file selection window. Once the file is selected, it can be seen in the SSAM files section of the ttest tab.
Figure 38. Screen Capture. SSAM Screen-Ttest Tab File Selection Window.
Figure 39 shows the case document file added to the SSAM file section, which is named 152& 56_ssam in this example. The user can click Delete to delete the added case document file or Compare opened file with to add another case document file. Since a t-test is used to compare the results of two case document files, only one other case document file can be added to the current case document for each analysis.
After another case document file is added, three sets of data must be configured in the t-test configuration section shown in figure 39 before the t-test analysis can begin.
The data to be configured include the following:
Figure 39. Screen Capture. SSAM Screen-Ttest Tab Default Settings.
As seen in figure 40, the t-test configuration section displays the following settings:
Figure 40. Screen Capture. SSAM Screen-Ttest Tab Modified Settings.
After both the SSAM file section and t-test configuration sections have been configured, click Apply. The added case document opens and becomes the active window.
Figure 41. Screen Capture. SSAM Screen-Ttest Tab Open Confirmation Message.
Figure 42. Screen Capture. SSAM Screen-Ttest Tab Active Case Document.
This is all the necessary data for the t-test analysis, which will compare the unfiltered conflicts data for these two case documents with t-test significant level 0.05 and f test significant level 0.1. By clicking Analyze, the tabular t-test results will be displayed in the result section, as shown in figure 43. The label has been changed to t-test for all data that have been applied.
Figure 43. Screen Capture. SSAM Screen-Ttest Tab Results Table.
The t-test results table has two subtables. Figure 44 provides a clear view of the table, which includes the following sections:
Figure 44. Screen Capture. SSAM Screen-Ttest Tab with Enlarged Results Table.
For each subtable, there are 11 columns. The first column lists the SSAM measures to be analyzed. For each measure, columns 2 through 4 list the corresponding mean, variance, and number of samples for the first case document. In this example, that document is 7692 yale road & hocking avenue_ssam. Columns 5 through 7 list the mean, variance, and number of samples for the second case document. In this example, that document is 152& 56_ssam. Column 8 shows the t value calculated for each measure. Column 9 displays the t critical value, obtained by looking at t tables. Column 10 shows whether or not each type of measure between these two case documents is significantly different. Column 11 shows the relative difference between the mean values of each measure.
Exporting the T-Test Results
The t-test results can be exported to a .csv file. Click Export to bring up the file menu shown in figure 45. By default, the exported file given the same name as the case document; in this example, the file is named 7692-yale road & hocking avenue_ssam_t-test.csv. Click Save to save the file.
Figure 45. Screen Capture. SSAM Screen-Ttest Tab Export T-Test Results.
Figure 46 shows the 7692-yale road & hocking avenue_ssam_t-test.csv file open in Microsoft ® Excel. The first row of the .csv file displays the column names as seen in the results table of the case document.
Figure 46. Screen Capture. Excel Screen-T-Test Results in .csv File.
Figure 47. Screen Capture. SSAM Screen-Ttest Tab Filtered Data Selected.
Figure 48. Screen Capture. SSAM Screen-Ttest Tab Filtering Confirmation Message.
After clicking OK, the user can click Analyze to continue the t-test process or return to the filtering screen for each case document to perform the desired filtering before continuing. If both case documents have filtered data, clicking Analyze will result in a t-test results table similar to figure 43. If one of the case documents does not have filtered data, a warning message will appear as shown in figure 49. This message displays, "There's no filtering data for one or two of the case files." The user must to go back to check on those two case files.
Using the Map Tab
SSAM contains a built-in feature to visualize conflicts with respect to type (crossing, rear-end, or lane changing) and a time-to-collision preset threshold. The conflicts can be visualized within a map or imported to SSAM as a graphical image (20+ formats supported) or VISSIM .inp file. Visualization through an .inp file is more user-friendly because, unlike in the graphical image, map coordinates are specified.
Figure 50. Screen Capture. SSAM Screen-Map Tab.
There are two sections in the Map tab-the Map Info and the Map View-which are described in the following subsections:
This section allows the user to load the map file or VISSIM .inp file and to configure the conflicts that will be displayed within the map. It has four subsections, shown in figure 51.
Figure 51. Screen Capture. SSAM Screen-Map Tab Map Info Section.
This section of the user interface, outlined in red in figure 52, is used to display the loaded map and to show the conflicts configured and filtered by the user within the map. Before the map is loaded, this section is empty.
Figure 52. Screen Capture. SSAM Screen-Map Tab Map View Section.
Showing Conflicts Without Loading Map Files
SSAM can show conflicts in the map view section without loading a map file as a background. This is done by clicking Show at the bottom of the Map Info section. Figure 53 shows the display of 318 conflicts in the map view section. No map file or map coordinates have been specified. Conflicts shown without loading a map file are only a view of conflict layout. To receive detailed conflict information (e.g., absolute location), the user must load the corresponding map file and specify the correct map coordinates, which is covered in the next section of the manual.
Figure 53. Screen Capture. SSAM Screen-Map Tab Conflicts Without Background.
Figure 54. Screen Capture. SSAM Screen-Map Tab Filtered Conflicts Without Background.
Showing Conflicts on Loaded Map Image
Users can load a map file to be used as a background displaying the conflicts within the current case document. Click the Edit map button which corresponds to the map label in the Map Info section to bring up a file selection menu. A single file can be selected from this menu. Figure 55 shows the file selection window with the label Import map image.
Figure 55. Screen Capture. SSAM Screen-Map Tab Import Map Image.
Once the map image file is selected, it can be seen in the map view section of the Map tab, as shown in figure 56. In this example, the selected map image file is named yale&hocking.bmp. The description label in the first row of the Map Info section has changed to yale&hocking.bmp, denoting that a map image file named has been loaded into the case document successfully.
Figure 56. Screen Capture. SSAM Screen-Map Tab with Map File Loaded.
The software will use the loaded map image as a background to show the analyzed conflicts. To locate those conflicts, the user must specify the (x, y) coordinates of the reference points within the map image. The user can specify (x, y) coordinates for two reference points: the upper-left and lower-right corners of the map image. To specify the coordinates, click Edit in the Coord section. This will result in the message shown in figure 57, which states, "Please provide the coordinates (x1, y1) and (x2, y2) for the upper-left and lower-right corners of the map image." Type all (integer or decimal) values on one line, separated by a single space.
Figure 57. Screen Capture. SSAM Screen-Map Tab Map Coordinates Input.
Figure 58. Screen Capture. SSAM Screen-Map Tab Map Coordinates Input Entered.
Figure 59 shows the input values recorded as two (x, y) coordinates. These coordinates replace the description label None Specified in the second row of the Map Info section. As shown in figure 59, conflicts are located within the intersection shown on the map image and the conflicts layout matches the intersection geometry. Each conflict dot represents a single conflict or multiple conflicts happening in the same place. Click each conflict dot to retrieve a display of the detailed information for the conflicts. The green rectangle surrounding all of the conflicts represents the conflicts location range specified by the Location Filter in the Filter tab.
Figure 59. Screen Capture. SSAM Screen-Map Tab Conflicts with Background.
Users can zoom in or out of the map view by clicking + or - in the Coord section. Figure 60 shows a zoomed in view of the conflicts and the map image. Click Fit to restore the default map view.
Figure 60. Screen Capture. SSAM Screen-Map Tab with Zoomed Map View.
Conflicts shown here are in sync with the conflicts specified by the conflicts table and summary table in the Filter tab. To only view conflicts with specified filtering criteria, users can perform the preferred filter operations in the Filter tab and then return to the Map panel. Filtered conflicts are shown in the map panel. Figure 61 shows only lane change conflicts with 1.0 = < TTC < = 1.5.
Figure 61. Screen Capture. SSAM Screen-Map Tab with Filtered Conflicts.
Showing Conflicts on Loaded VISSIM .inp File
Users can load a VISSIM .inp file to generate the corresponding .trj file processed by SSAM. Click Edit inp in the Map Info section to retrieve a file selection menu through which a single file can be selected. Figure 62 shows the file selection window. In this example, the file selected is yale&hocking.inp. Click Open to import the file.
Figure 62. Screen Capture. SSAM Screen-Map Tab Load .inp File.
The Coord Edit button is inactive as the user will not provide the coordinates for the loaded .inp file. After loading the .inp file, the network displays in the map view section, as shown in figure 63.
Figure 63. Screen Capture. SSAM Screen-Map Tab Loaded .inp File and Generated Network.
The loaded .inp file contains all the needed information for the network. Conflicts generated from this network are shown once the .inp file is loaded. A zoomed in view of the network is shown in figure 64.
Figure 64. Screen Capture. SSAM Screen-Map Tab with Zoomed View of Generated Network.
Saving Image File
Users can export the image shown in the map view section by clicking Save image at the bottom of the Map Info section. As shown in figure 65, clicking Save image will retrieve a file selection window labeled Save Image. SSAM allows the image to be save in .png format only. In this example, the image is saved as 7692-yale & hocking avenuw_ssam.png. Figure 66 shows the resulting .png file displayed using Microsoft® Office Picture Manager.
Filtering Conflicts by Area
In SSAM, the user can filter conflicts in the Filter tab and also filter conflicts by area in the Map tab. To filter in the Map tab, the user will drag a rectangle to specify the area of interest as shown in figure 67.
Figure 67. Screen Capture. SSAM Screen-Map Tab Specify Rectangle.
Figure 68. Screen Capture. SSAM Screen-Map Tab Filter Confirmation Message.
Figure 69. Screen Capture. SSAM Screen-Map Tab Filtered Conflicts.
Configuring Conflict Display
After a map file is loaded into the case document, users can specify how different conflicts within the map file are displayed. As shown in figure 71, there are two types of settings to configure. The first part of configuration allows the user to choose the icon shape for different types of conflicts-crossing, rear-end, and lane-change. There are three icon shapes for each type of conflicts-oval, rectangular, and triangle. The second part of the configuration allows the user to select display colors for conflicts with different severity levels. Severity levels are classified according to different group of TTC values. They are crash conflicts with TTC less and equal to 0.5 second, conflicts with TTC less and equal to 1.0 second, and conflict with TTC less and equal to 1.5 seconds.
Figure 71 shows crossing conflicts with the oval icon, rear-end conflicts with the rectangular icon, and lane-change conflicts with the triangle icon. It also shows crashes in blue, conflicts with TTC < = 0.5 in red, conflict with TTC < = 1.0 in orange, and conflicts with TTC < = 1.5 in green. The user can enable the conflict display by clicking Show. Conflicts with the user-defined configurations will be shown in the map view window.
Figure 71. Screen Capture. SSAM Screen-Map Tab Conflicts Display Configuration.
Figure 72. Screen Capture. SSAM Screen-Map Tab Additional Conflicts Display Configuration.
Displaying Conflict Line
Users can receive detailed conflict information for each conflict point shown in the conflict map view, as shown in figure 73. There are two sections within the resulting window. The right section provides the detailed surrogate safety measures corresponding to this conflict, including TTC, PET, as well as vehicle IDs and lane/link IDs. The left section shows the conflict lines for the two vehicles involved in this conflict. A blue line represents the first vehicle and a red line represents the second vehicle. Both conflict lines have arrows to indicate movement direction.
Recoding Map Coordinates
Users can get the corner point coordinates of a .bmp file from a VISSIM (or others network. Figure 74 shows a VISSIM network file (.inp file) named 7692-yale road & hocking avenue.inp, which is the network that generates the .trj files used for the example in this manual.
Figure 74. Screen Capture. VISSIM Screen-Network Example.
Map files can be generated by the print screen operation on this VISSIM network (or other programs). Users can paste in Print and save the image as .bmp file. The map corner coordinates can then be recorded within the VISSIM network, as shown in figure 75 and figure 76. The user can move the mouse cursor to the upper-left corner and lower-right corner of the network and record the coordinates data located at the bottom of the network in (x, y) format.
Figure 76. Screen Capture. VISSIM Screen- Record Lower-Right Corner Coordinates.
Saving the Case Document
To save the case document, click Save. Users can also choose Save in the File menu. In figure 77, the case document has not yet been saved, as indicated by the (unsaved) text appended to the case name in the title bar. In figure 78, the case document has been saved.
Figure 77. Screen Capture. SSAM Screen-Case Document Not Saved.
Figure 78. Screen Capture. SSAM Screen-Case Document Saved.
The case document can now be closed in the internal window. The case document remains in the workspace window after closing the case document as shown in figure 79.
Figure 79. Screen Capture. SSAM Screen-Case Document Closed.
The case document can be opened again either by double-clicking on the case document name in the workspace window or by right-clicking on the case document name and choosing Open from the resulting menu. Choosing Delete from this menu will remove the document from the workspace and delete the case document from the disk.
Editing an Existing Case Document
Figure 80. Screen Capture. SSAM Screen-Open Case Document from Workspace.
Figure 81. Screen Capture. SSAM Screen-Confirm Delete Message.
The message informs the user that all the data in the Conflict tab and Summary tab will be deleted. This means the Case Files section of a case document cannot be edited without discarding the conflict and summary information stored in the old document.
Figure 82. Screen Capture. SSAM Screen-Edit a Case Document.
Figure 83. Screen Capture. SSAM Screen-Delete a .trj File.
Click Apply to apply the changes to the case document. This will activate the Analyze button. The case document is now ready to be analyzed again with the new case files. Note that applying changes does not save the case document. In order to save the case document, users must click Save or choose the Save option in the File menu.
Checking Filter Robustness
SSAM is robust in terms of checking user input errors. For example, if a user attempts to close an unsaved case document, the software will prompt the user to save the case document to disk. This section describes the robust features provided by the SSAM software filter. SSAM offers a robust filter which makes sure that the user sets the valid filters.
To apply a filter, click Apply Filter. When this button is clicked, SSAM checks if there is any data to which the filter can be applied. If the user attempts to apply a filter to nonexisting data, the software will inform the user that a case document must first be populated with conflict data which can be filtered. Figure 84 shows the error message for this user input error.
Figure 84. Screen Capture. SSAM Screen-Error Message for Nonexisting Data.
If the case document has been populated with conflict data, the SSAM software checks for the following user input errors:
Figure 85. Screen Capture. SSAM Screen-Filter Tab Errors.
In figure 85 we can see the following errors in the filter settings:
Figure 86. Screen Capture. SSAM Screen-Wrong Filter Settings Error Message.
Null Result Filter
Sometimes the filter settings applied by the user might not yield a result. For example, if there are no lane-change conflicts and the user wants to view only lane-change conflicts, no conflicts will be shown in the Conflict tab. In this case, the SSAM software will inform the user that the filter did not yield any results. However, the data in the Conflict tab and the Summary tab will remain unchanged. Figure 87 shows the message for this event.
This section will demonstrate the SSAM software's ability to present documents in different ways. It will also describe the added functionality of exporting data from SSAM to a .csv format which can be viewed in Microsoft® Excel.
In this example, two case documents-140 & 72_ssam and 184 & fraser_ssam-are open. Figure 88 shows three case documents in the workspace area and 140 & 72_ssam open.
Figure 88. Screen Capture. SSAM Screen-Opening Case Documents.
The relevant information for the selected case document is seen in the internal frame. Notice in figure 88 that the name of the internal frame is the same as the name of the selected file. The SSAM software allows the user to view all open case documents at once. The user can tile the case documents in the following ways:
Using the Horizontal Tiling
Figure 89. Screen Capture. SSAM Screen-Horizontal Tiling.
Using the Vertical Tiling
Figure 90. Screen Capture. SSAM Screen-Vertical Tiling.
Using the Grid Tiling
Figure 91. Screen Capture. SSAM Screen-Grid Tiling.
Using the Cascade Tiling
Figure 92. Screen Capture. SSAM Screen-Cascade Tiling.
All the documents must be opened to be seen in a tiled view. Each of the case document frames can be resized; however, the elements inside the internal window of a case document will not be resized according to the size of the frame. This can be seen in the figures above.
Toggling the Workspace Window
SSAM also allows the user to toggling the workspace. This can be done by clicking Toggle Workspace on the toolbar or selecting Toggle Workspace in the Window menu. Figure 93 shows the tool when the workspace window is toggled out.
Figure 93. Screen Capture. SSAM Screen-Toggling the Workspace Window.
SSAM allows the user to export the conflict data from the Conflict tab into a .csv file. Click Export to export the conflict data to a .csv file. Clicking the Export button will result in the file menu shown in figure 94. In this example, the export file is named export.csv. Click Save to save the file.
Figure 94. Screen Capture. Export Conflict Table to .csv File.
Figure 95 shows the export.csv file when it is open in Microsoft® Excel. The first row of the .csv file is marked in figure 95 to show the column names as seen in the Conflict tab of the case document.
Figure 95. Screen Capture. Microsoft® Excel-Exported .csv File.
APPENDIX. Terms and Definitions
This appendix serves as a glossary, defining several surrogate safety measures and other terms used within SSAM.
Case document is a file which contains the required information for analyzing trajectory data. It also contains the result set of the analysis. The required information for creating a case document is as follows:
All of the above are configured via the Configuration tab in the SSAM software.
The result set obtained after analyzing a single or a group of trajectory files in the case document comprises of the following information:
A case document can be overwritten and saved.
The following surrogate safety measures are ordered according to their column-wise appearance (from left-to-right) in the conflicts table on the Conflicts tab.
trjFile is the name of the trajectory file (i.e.,.trj file) where this conflict was identified.
tMinTTC is the simulation time where the minimum TTC (time-to-collision) value for this conflict was observed. At the beginning of each simulation, the time is (generally) 0 and advances in increments of 0.1 to 1.0 seconds per time step depending on the resolution of the simulation software.
xMinPET is the x-coordinate specifying the approximate location of the conflict at the time when the minimum PET (post encroachment time) was observed. More specifically, this location corresponds to the center of the (first) vehicle, where the subsequent arrival of the second vehicle to the same location was the shortest encroachment observed.
yMinPET is the y-coordinate specifying the approximate location of the conflict at the time when the minimum PET (post encroachment time) was observed. More specifically, this location corresponds to the center of the (first) vehicle, where the subsequent arrival of the second vehicle to the same location was the shortest encroachment observed.
TTC is the minimum time-to-collision value observed during the conflict. This estimate is based on the current location, speed, and trajectory of two vehicles at a given instant.
PET is the minimum post encroachment time observed during the conflict. Post encroachment time is the time between when the first vehicle last occupied a position and the second vehicle subsequently arrived at the same position. A value of 0 indicates an actual collision.
MaxS is the maximum speed of either vehicle throughout the conflict (i.e., while the TTC is less than the specified threshold). This value is expressed in feet per second or meters per second, depending on the units specified in the corresponding trajectory file.
DeltaS is the difference in vehicle speeds as observed at tMinTTC. More precisely, this value is mathematically defined as the magnitude of the difference in vehicle velocities (or trajectories), such that if v1 and v2 are the velocity vectors of the first and second vehicles respectively, then DeltaS = || v1 - v2 ||. Consider an example where both vehicles are traveling at the same speed, v. If they are traveling in the same direction, DeltaS = 0. If they have a perpendicular crossing path, DeltaS = (v2)v. If they are approaching each other head on, DeltaS = 2v.
DR is the initial deceleration rate of the second vehicle. Note that in actuality, this value is recorded as the instantaneous acceleration rate. If the vehicle brakes (i.e., reacts), this is the first negative acceleration value observed during the conflict. If the vehicle does not brake, this is the lowest acceleration value observed during the conflict. This value is expressed in feet per second or meters per second, depending on the units specified in the corresponding trajectory file.
MaxD is the maximum deceleration of the second vehicle. Note that in actuality, this value is recorded as the minimum instantaneous acceleration rate observed during the conflict. A negative value indicates deceleration (braking or release of gas pedal). A positive value indicates that the vehicle did not decelerate during the conflict. This value is expressed in feet per second or meters per second, depending on the units specified in the corresponding trajectory file.
MaxDeltaV is the maximum DeltaV value of either vehicle in the conflict (see FirstDeltaV or SecondDeltaV for more information).
ConflictAngle is an approximate angle of hypothetical collision between conflicting vehicles, based on the estimated heading of the each vehicle (see explanation of FirstHeading). The angle, expressed in the perspective of the first vehicle to arrive at the conflict point, conveys the direction from which the second vehicle is approaching the first vehicle, as shown in figure 96. The angle ranges from -180 ° to +180 °, where a negative angle indicates approach from the left and a positive angle indicates approach from the right. An angle of 180 ° (or -180 °) indicates a direct head-on approach, and an angle of 0 ° (or -0 °) indicates a direct rear approach.
Figure 96. Illustration. Conflict Angle.
ClockAngle is an alternative expression of the conflict angle in terms of a more familiar clock hand positions. Again, the angle is expressed in the perspective of the first vehicle, with the clock time indicating the angle from which the second vehicle is approaching, as shown in figure 97. The 12:00 position is direct ahead of the first vehicle, 3:00 is to the right, 6:00 is directly behind, and 9:00 is to the left.
Figure 97. Illustration. Clock Angle.
ConflictType describes whether the conflict is the result of a rear end, lane change, or crossing movement. If link and lane information is not available for both vehicles, then the event type is classified based solely on the absolute value of the ConflictAngle as follows. The type is classified as a rear-end conflict if ||ConflictAngle|| < 30 °, a crossing conflict if ||ConflictAngle|| > 85 °, or otherwise a lane-changing conflict. However, the simulation model that produced the vehicle trajectory data can generally provide link and lane information for both vehicles-though the coding of these values may vary significantly from one simulation vendor to the next. If link and lane information is available, that information is utilized for classification in the case that the vehicles both occupy the same lane (of the same link) at either the start or end of the conflict event. If the vehicles both occupy the same lane at the start and end of the event, then it is classified as a rear-end event. If either vehicle ends the conflict event in a different lane than it started (while having not changed links), then the event is classified as a lane-change. If either of the vehicles changes links over the course of the event, then the conflict angle determines the classification as previously described, with the following possible exception). For two vehicles that begin the conflict event in the same lane, but change links over the course of the event, the classification logic considers only rear-end or lane-change types, based on the conflict angle (using the threshold value previously mentioned). Note that vehicle maneuvers such as changing lanes into an adjacent turn bay lane or entering into an intersection area may be consider changing links, depending on the underlying simulation model. In some cases, vehicles which appear to be traveling in the same lane may actually by considered by the simulation model as traveling on different links that happen to overlap.
PostCrashV is an estimate of the post collision velocity of both vehicles. This estimate assumes that the vehicles did crash, at the estimated ConflictAngle, at velocities observed at the tMinTTC, and assuming an inelastic collision between the center of mass of both vehicles, where both vehicles subsequently deflect in the same direction and at the same velocity.
PostCrashHeading is the estimated heading of both vehicles following a hypothetical collision (as discussed in PostCrashV). This heading is expressed as the angle measured counterclockwise from the x-axis (which is assumed to point right), such that 0 ° is right, 90 ° is up, 180 ° is left, and 270 ° is down. The angle ranges from 0 ° to 360 °.
FirstVID (SecondVID) is the vehicle identification number of the first (second) vehicle. The first vehicle is the vehicle that arrives to the conflict point first. The second vehicle subsequently arrives to the same location. In rare cases (actually collisions) both vehicles may arrive to a location simultaneously, in which case the tie between first and second vehicle in broken arbitrarily.
FirstLink (SecondLink) is a number indicating which link the first (second) vehicle is traveling on at tMinTTC.
FirstLane (SecondLane) is a number indicating in which lane the first (second) vehicle is traveling on at tMinTTC.
FirstLength (SecondLength) is the length of the first (second) vehicle in feet or meters.
FirstWidth (SecondWidth) is the width of the first (second) vehicle in feet or meters.
FirstHeading (SecondHeading) is the heading of the first (second) vehicle during the conflict. This heading is approximated by the change in position from the start of the conflict to the end of the conflict. Note that in most non-rear-end conflicts, at least one vehicle is turning throughout the conflict. Its actual heading would vary accordingly throughout the conflict. If the vehicle does not move during the conflict, then the direction in which it is facing is taken as the heading. This heading is expressed as the angle measured counterclockwise from the x-axis (which is assumed to point right), such that 0 ° is right, 90 ° is up, 180 ° is left, and 270 ° is down. The angle ranges from 0 ° to 360 °.
FirstVMinTTC (SecondVMinTTC) is the velocity (speed) of the first (second) vehicle at tMinTTC.
FirstDeltaV (SecondDeltaV) is the change between conflict velocity (given by speed FirstVMinTTC and heading FirstHeading) and the post-collision velocity (given by speed PostCrashV and headingPostCrashHeading). This is a surrogate for the severity of the conflict, calculated assuming a hypothetical collision of the two vehicles in the conflict.
xFirstCSP (xSecondCSP) is the x-coordinate of the first (second) vehicle at the conflict starting point (CSP). The CSP is the location of the vehicle at tMinTTC.
yFirstCSP (ySecondCSP) is the y-coordinate of the first (second) vehicle at the conflict starting point (CSP). The CSP is the location of the vehicle at tMinTTC.
xFirstCEP (xSecondCEP) is the x-coordinate of the first (second) vehicle at the conflict ending point (CEP). The CEP is the location of the vehicle at either the last time step where the TTC value is below the specified threshold or where the last post-encroachment value was observed, whichever occurs later in the conflict timeline.
yFirstCEP (ySecondCEP) is the y-coordinate of the first (second) vehicle at the conflict ending point (CEP). The CEP is the location of the vehicle at either the last time step where the TTC value is below the specified threshold or where the last post-encroachment value was observed, whichever occurs later in the conflict timeline.
 Some issues with proper specification of units have been noted in certain versions of VISSIM (version 4 series), in which case use of metric units is suggested with version 4 to avoid potential issues.
Topics: research, safety, data and analysis tools
Keywords: research, safety, simulation, surrogate measures, conflict, software, user manual
TRT Terms: highway safety, simulation, traffic conflicts