Frequently Asked Questions (FAQs)

The following FAQs were developed to assist the practitioner in properly selecting and applying traffic analysis tools to support future traffic operations analyses.

How should the project scope and physical limits be established?
Which traffic analysis tool is best?
How do I pick a traffic analysis tool?
Do traffic microsimulation tools replace the 2000 Highway Capacity Manual (HCM) or associated software implementations of the HCM (e.g., Highway Capacity Software and HiCAP-2000)?
The freeway and arterial network being analyzed is congested today, and it will also be congested tomorrow. Why should I consider the HCM to be a cornerstone tool?
How does the output of a microsimulation tool compare to the Level of Service output of the HCM?
Given the same input data, why is the output of the HCM and that of a traffic microsimulation tool different?
Why is the level of effort to apply simulation greater than applying the HCM?
Why are there so many more input parameters in a microsimulation tool than there are in the HCM 2000?
Why would a project manager, planner or engineer want to commit the time and resources needed to effectively apply microsimulation?
How does the coding of the network affect the output of a microsimulation tool?
Why should I conduct multiple runs with a microsimulation analysis?
How many runs should I complete when using a microsimulation tool?
What type of documentation is required for a microsimulation analysis?

Question: How should the project scope and physical limits be established?

Answer: The limits and scope of a project must be established with a consideration of the operational and geometric characteristics of the existing, no build future, and future build alternatives and the traffic analysis tool being used. Complex urban environments that include congested roadways require the analyst to look beyond the physical construction limits of a potential project in order to fully capture the impact of project alternatives on the extent of the congestion. This expanded assessment supports an informed decision on how the proposed localized alternative impacts the operations of these adjacent roadways.

Question: Which traffic analysis tool is best?

Answer: It depends on the particular application. Specifically, each tool has a suite of advantages and disadvantages, limitations, and capabilities that must be considered by the analyst. A tool that is very powerful and responsive to the analysis of a freeway may not be appropriate for the analysis of a roundabout. For this reason, the analyst and project manager must consider the purpose, need, and scope of the given problem and select a tool which supports the project requirements. More information about selecting the proper traffic analysis tool be found on Selection of the Proper Tool.

Question: How do I pick a traffic analysis tool?

Answer: Volume II of the FHWA Traffic Analysis Toolbox aids in the assessment and selection process of a tool. You can also refer to published research, technical papers, and user communities of various tools. It is essential to understand and select an appropriate tool based upon its limitations and capabilities.

Question: Do traffic microsimulation tools replace the 2000 Highway Capacity Manual (HCM) or associated software implementations of the HCM (e.g., Highway Capacity Software and HiCAP-2000)?

Answer: Traffic microsimulation tools should not be used as a replacement to the HCM. The HCM remains widely accepted throughout the transportation profession as a credible tool. The FHWA Traffic Analysis Toolbox is designed on the premise that the most appropriate analysis tool should be selected to support the analysis. The analysis process may also require the use of a series of tools from the initial sketch planning stage to a more complete operational analysis to refine a proposed design.

Acknowledged throughout the HCM, limitations exist in the methodology. Volume II of the FHWA Traffic Analysis Toolbox provides a brief overview of these limitations. To overcome these limitations, analysts are utilizing traffic simulation to compliment the project development process. It is critical however that the limitations of these additional tools are also understood and accounted for in the application.

Question: The freeway and arterial network being analyzed is congested today, and it will also be congested tomorrow. Why should I consider the HCM to be a cornerstone tool?

Answer: The HCM can evaluate the impacts of many congested network scenarios, such as on a freeway network or isolated intersections as long as the effect, limitations, and context of the results is understood. Yet, other congested scenarios are beyond the limitations of the HCM, such as signalized arterial networks or freeway-arterial interfaces. However, HCM can still be useful in the scenarios beyond its limits. For instance, the HCM enables the analyst to quickly assess the existing conditions of a given location, confirm “hot spot” locations, and compared to the complexity of microsimulation, is an easy tool to aid in the realization of the cause and effects of modified geometry and operational schemes. Additionally, the use of the HCM may aid in the establishment of a microsimulation analysis purpose, need, scope and physical network limits.

Question: How does the output of a microsimulation tool compare to the Level of Service output of the HCM?

Answer: The comparison of microsimulation analyses outputs (such as average delay or density) and the associated output measure of the HCM should not be directly correlated if the analysis methodology of the microsimulation tool is not the same as the HCM methodology. It must also be acknowledged that the level of calibration or reliance on assumptions between tools will influence the respective outputs. Therefore, attempts to judge results from competing methodology should only be considered with a solid appreciation of the differences in the tools.

As the analyst conducts the operations analysis with a tool other than the HCM, it is essential that the output be reported in a manner that is consistent with the methodology of the tool developer. Post processing of output data from a microsimulation tool with the methodology of the HCM is one approach that overcomes this discrepancy between methodologies.

Question: Given the same input data, why is the output of the HCM and that of a traffic microsimulation tool different?

Answer: A number of factors can attribute to the discrepancies. This may vary between tools, but overall may be due to the respective tools ability to analyze the alternative, assumptions made due to gaps in data, the level of effort dedicated to calibrating the model, and the way in which the respective tool accumulates and reports output data.

Question: Why is the level of effort to apply simulation greater than applying the HCM?

Answer: There are a number of factors that impact the time and effort in applying simulation tools. In summary, it is due to the time and complexity in collecting adequate field data, setting up and coding a base model, adequately calibrating the base model, and performing multiple runs for various improvement alternatives. A detailed discussion is contained in Volume III of the FHWA Traffic Analysis Toolbox.

Question: Why are there so many more input parameters in a microsimulation tool than there are in the HCM 2000?

Answer: Due to the nature of a microsimulation tool, the data requirements exceed that of the HCM 2000 in order to support the underlying algorithms used to model the conditions of the respective network. In applying any microsimulation tool, the analyst should understand the tools reliance on the user defined and default parameters, the random seed parameters, as well as the sensitivity of each upon the final output. By understanding this, the analyst quickly realizes that simulation is an extremely powerful tool, but is also easily manipulated or misapplied to create unrealistic operational conditions. Because of this, the analyst is challenged to calibrate the tool by modifying the input parameters in the time and within the budget allocated. It is also essential that appropriate level of input data is considered and assumptions are made commensurate to the problem statement and scope of the project. Additional information is contained in Volume III of the FHWA Traffic Analysis Toolbox.

Question: Why would a project manager, planner or engineer want to commit the time and resources needed to effectively apply microsimulation?

Answer: Realizing that the HCM has limitations, the Federal Highway Administration, partnering agencies, and practitioners are complimenting their use of the HCM with other complimentary traffic analysis tools. This is driven by a desire to better understand and support a robust operations analysis and administrative decision. One recurring example in which additional tools are being considered is for scenarios where one or more features of an interchange or corridor operate in unstable or over-saturated conditions for a given design period, such as the afternoon rush “hour”. Due to the interdependence of the individual features of a corridor or interchange, microsimulation increases the understanding of how an alternative may function in the future.

Question: How does the coding of the network affect the output of a microsimulation tool?

Answer: This varies on the tool and it is suggested that the manufacturer or vendor be consulted to understand the details and sensitivity of a respective product. It however is important to understand that the placement and coding of links, nodes and connectors in certain combinations, order and spacing may impact the reported measure of effectiveness for a given location in the network. While two models representing the same network may have similar global operating outputs and animation, a specific link, node or connector may have drastically different numerical output due to how the network was coded.

Question: Why should I conduct multiple runs with a microsimulation analysis?

Answer: Microsimulation tools utilize algorithms that consider and reflect the interaction of individual vehicles throughout the given roadway network. As a Stochastic Tool, microsimulation tools assign probabilities to many of the decisions drivers make on a sub-second level (for example; whether or not to make a lane change) for the purpose of better reflecting the randomness inherent in the field. Random numbers are generated within the microsimulation tool to account for the fact that drivers do not always make the same decisions under the same conditions. As a result, a fixed set of assumptions and known conditions could generate different output results in separate runs. To account for this, an analyst should perform multiple runs using the same assumptions and conditions, along with a statistical analysis to increase their confidence of the overall analysis results. Single runs that are not representative of the random nature of these tools will reduce the credibility of the analysis and reduce the acceptance of the results.

Question: How many runs should I complete when using a microsimulation tool?

Answer: Unfortunately, there is no single answer. The minimum number of runs required depends on a number of factors, including the variance of the measure of effectiveness, required confidence level, and minimum acceptable error. Detailed information regarding the number of runs is contained in Volume III of the Traffic Analysis Toolbox.

Question: What type of documentation is required for a microsimulation analysis?

Answer: The level and degree of documentation in the past, as well as the methodology in applying microsimulation has varied greatly. To advance the state of the practice, Volume III of the FHWA Traffic Analysis Toolbox outlines a process that may be referenced by project mangers and analysts as they approach and conduct a project. Based on this, the respective documentation may be developed commensurate to the needs of the agencies and the scope of the project. A series of microsimulation case studies are currently being developed by FHWA and will be available in Fall 2005. Several state agencies are in the process of establishing their requirements regarding process and documentation for simulation and may be considered to support new or evolving applications.

Search Operations: Traffic Analysis Tools Home About the Program Traffic Analysis Tools Team Types of Traffic Analysis Tools FAQs FHWA Products Traffic Analysis Toolbox Next-Generation Simulation (NGSIM) Dynamic Traffic Assignment (DTA) Corridor Simulation (CORSIM/TSIS) ITS Deployment Analysis System (IDAS) Resources Best Practices Training & Workshops Links Contact Us	Frequently Asked Questions (FAQs) Traffic Analysis Tools The following FAQs were developed to assist the practitioner in properly selecting and applying traffic analysis tools to support future traffic operations analyses. How should the project scope and physical limits be established? Which traffic analysis tool is best? How do I pick a traffic analysis tool? Do traffic microsimulation tools replace the 2000 Highway Capacity Manual (HCM) or associated software implementations of the HCM (e.g., Highway Capacity Software and HiCAP-2000)? The freeway and arterial network being analyzed is congested today, and it will also be congested tomorrow. Why should I consider the HCM to be a cornerstone tool? How does the output of a microsimulation tool compare to the Level of Service output of the HCM? Given the same input data, why is the output of the HCM and that of a traffic microsimulation tool different? Why is the level of effort to apply simulation greater than applying the HCM? Why are there so many more input parameters in a microsimulation tool than there are in the HCM 2000? Why would a project manager, planner or engineer want to commit the time and resources needed to effectively apply microsimulation? How does the coding of the network affect the output of a microsimulation tool? Why should I conduct multiple runs with a microsimulation analysis? How many runs should I complete when using a microsimulation tool? What type of documentation is required for a microsimulation analysis? Question: How should the project scope and physical limits be established? Answer: The limits and scope of a project must be established with a consideration of the operational and geometric characteristics of the existing, no build future, and future build alternatives and the traffic analysis tool being used. Complex urban environments that include congested roadways require the analyst to look beyond the physical construction limits of a potential project in order to fully capture the impact of project alternatives on the extent of the congestion. This expanded assessment supports an informed decision on how the proposed localized alternative impacts the operations of these adjacent roadways. Back to Top Question: Which traffic analysis tool is best? Answer: It depends on the particular application. Specifically, each tool has a suite of advantages and disadvantages, limitations, and capabilities that must be considered by the analyst. A tool that is very powerful and responsive to the analysis of a freeway may not be appropriate for the analysis of a roundabout. For this reason, the analyst and project manager must consider the purpose, need, and scope of the given problem and select a tool which supports the project requirements. More information about selecting the proper traffic analysis tool be found on Selection of the Proper Tool. Back to Top Question: How do I pick a traffic analysis tool? Answer: Volume II of the FHWA Traffic Analysis Toolbox aids in the assessment and selection process of a tool. You can also refer to published research, technical papers, and user communities of various tools. It is essential to understand and select an appropriate tool based upon its limitations and capabilities. Back to Top Question: Do traffic microsimulation tools replace the 2000 Highway Capacity Manual (HCM) or associated software implementations of the HCM (e.g., Highway Capacity Software and HiCAP-2000)? Answer: Traffic microsimulation tools should not be used as a replacement to the HCM. The HCM remains widely accepted throughout the transportation profession as a credible tool. The FHWA Traffic Analysis Toolbox is designed on the premise that the most appropriate analysis tool should be selected to support the analysis. The analysis process may also require the use of a series of tools from the initial sketch planning stage to a more complete operational analysis to refine a proposed design. Acknowledged throughout the HCM, limitations exist in the methodology. Volume II of the FHWA Traffic Analysis Toolbox provides a brief overview of these limitations. To overcome these limitations, analysts are utilizing traffic simulation to compliment the project development process. It is critical however that the limitations of these additional tools are also understood and accounted for in the application. Back to Top Question: The freeway and arterial network being analyzed is congested today, and it will also be congested tomorrow. Why should I consider the HCM to be a cornerstone tool? Answer: The HCM can evaluate the impacts of many congested network scenarios, such as on a freeway network or isolated intersections as long as the effect, limitations, and context of the results is understood. Yet, other congested scenarios are beyond the limitations of the HCM, such as signalized arterial networks or freeway-arterial interfaces. However, HCM can still be useful in the scenarios beyond its limits. For instance, the HCM enables the analyst to quickly assess the existing conditions of a given location, confirm “hot spot” locations, and compared to the complexity of microsimulation, is an easy tool to aid in the realization of the cause and effects of modified geometry and operational schemes. Additionally, the use of the HCM may aid in the establishment of a microsimulation analysis purpose, need, scope and physical network limits. Back to Top Question: How does the output of a microsimulation tool compare to the Level of Service output of the HCM? Answer: The comparison of microsimulation analyses outputs (such as average delay or density) and the associated output measure of the HCM should not be directly correlated if the analysis methodology of the microsimulation tool is not the same as the HCM methodology. It must also be acknowledged that the level of calibration or reliance on assumptions between tools will influence the respective outputs. Therefore, attempts to judge results from competing methodology should only be considered with a solid appreciation of the differences in the tools. As the analyst conducts the operations analysis with a tool other than the HCM, it is essential that the output be reported in a manner that is consistent with the methodology of the tool developer. Post processing of output data from a microsimulation tool with the methodology of the HCM is one approach that overcomes this discrepancy between methodologies. Back to Top Question: Given the same input data, why is the output of the HCM and that of a traffic microsimulation tool different? Answer: A number of factors can attribute to the discrepancies. This may vary between tools, but overall may be due to the respective tools ability to analyze the alternative, assumptions made due to gaps in data, the level of effort dedicated to calibrating the model, and the way in which the respective tool accumulates and reports output data. Back to Top Question: Why is the level of effort to apply simulation greater than applying the HCM? Answer: There are a number of factors that impact the time and effort in applying simulation tools. In summary, it is due to the time and complexity in collecting adequate field data, setting up and coding a base model, adequately calibrating the base model, and performing multiple runs for various improvement alternatives. A detailed discussion is contained in Volume III of the FHWA Traffic Analysis Toolbox. Back to Top Question: Why are there so many more input parameters in a microsimulation tool than there are in the HCM 2000? Answer: Due to the nature of a microsimulation tool, the data requirements exceed that of the HCM 2000 in order to support the underlying algorithms used to model the conditions of the respective network. In applying any microsimulation tool, the analyst should understand the tools reliance on the user defined and default parameters, the random seed parameters, as well as the sensitivity of each upon the final output. By understanding this, the analyst quickly realizes that simulation is an extremely powerful tool, but is also easily manipulated or misapplied to create unrealistic operational conditions. Because of this, the analyst is challenged to calibrate the tool by modifying the input parameters in the time and within the budget allocated. It is also essential that appropriate level of input data is considered and assumptions are made commensurate to the problem statement and scope of the project. Additional information is contained in Volume III of the FHWA Traffic Analysis Toolbox. Back to Top Question: Why would a project manager, planner or engineer want to commit the time and resources needed to effectively apply microsimulation? Answer: Realizing that the HCM has limitations, the Federal Highway Administration, partnering agencies, and practitioners are complimenting their use of the HCM with other complimentary traffic analysis tools. This is driven by a desire to better understand and support a robust operations analysis and administrative decision. One recurring example in which additional tools are being considered is for scenarios where one or more features of an interchange or corridor operate in unstable or over-saturated conditions for a given design period, such as the afternoon rush “hour”. Due to the interdependence of the individual features of a corridor or interchange, microsimulation increases the understanding of how an alternative may function in the future. Back to Top Question: How does the coding of the network affect the output of a microsimulation tool? Answer: This varies on the tool and it is suggested that the manufacturer or vendor be consulted to understand the details and sensitivity of a respective product. It however is important to understand that the placement and coding of links, nodes and connectors in certain combinations, order and spacing may impact the reported measure of effectiveness for a given location in the network. While two models representing the same network may have similar global operating outputs and animation, a specific link, node or connector may have drastically different numerical output due to how the network was coded. Back to Top Question: Why should I conduct multiple runs with a microsimulation analysis? Answer: Microsimulation tools utilize algorithms that consider and reflect the interaction of individual vehicles throughout the given roadway network. As a Stochastic Tool, microsimulation tools assign probabilities to many of the decisions drivers make on a sub-second level (for example; whether or not to make a lane change) for the purpose of better reflecting the randomness inherent in the field. Random numbers are generated within the microsimulation tool to account for the fact that drivers do not always make the same decisions under the same conditions. As a result, a fixed set of assumptions and known conditions could generate different output results in separate runs. To account for this, an analyst should perform multiple runs using the same assumptions and conditions, along with a statistical analysis to increase their confidence of the overall analysis results. Single runs that are not representative of the random nature of these tools will reduce the credibility of the analysis and reduce the acceptance of the results. Back to Top Question: How many runs should I complete when using a microsimulation tool? Answer: Unfortunately, there is no single answer. The minimum number of runs required depends on a number of factors, including the variance of the measure of effectiveness, required confidence level, and minimum acceptable error. Detailed information regarding the number of runs is contained in Volume III of the Traffic Analysis Toolbox. Back to Top Question: What type of documentation is required for a microsimulation analysis? Answer: The level and degree of documentation in the past, as well as the methodology in applying microsimulation has varied greatly. To advance the state of the practice, Volume III of the FHWA Traffic Analysis Toolbox outlines a process that may be referenced by project mangers and analysts as they approach and conduct a project. Based on this, the respective documentation may be developed commensurate to the needs of the agencies and the scope of the project. A series of microsimulation case studies are currently being developed by FHWA and will be available in Fall 2005. Several state agencies are in the process of establishing their requirements regarding process and documentation for simulation and may be considered to support new or evolving applications. Back to Top
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