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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Answer: The primer presents a high-level overview of the different types of traffic analysis tools and their role in transportation analyses. The target audiences are agency management and high level decision makers. Guidance Volume 1 Home Page Listing
Answer: This volume identifies key criteria and circumstances to consider when selecting the most appropriate type of traffic analysis tool for the analysis at hand. The audience for this volume are project managers and project engineers. Guidance Volume 2 Home Page Listing
Answer: These guidelines provide a recommended process for using traffic microsimulation software in traffic analyses. The audience for this volume are project managers and project engineers. Guidance Volume 3 Home Page Listing
Answer: This volume describes a process and acts as guidelines for the recommended use of CORSIM traffic simulation software in transportation analyses. The seven-step process presented in these guidelines highlights the aspects of a CORSIM analysis from project start to project completion. The seven steps in the process include: 1) scope project, 2) data collection, 3) base model development, 4) error checking, 5) calibration - comparing model MOEs to field data (and adjusting model parameters), 6) alternatives analysis, and 7) final report. Each step is described in detail and an example problem a plying the process is carried through the entire document. The audience for this volume are project managers and project engineers. Guidance Volume 4 Home Page Listing
Answer: The purpose of this Traffic Analysis Toolbox is to give the reader a summary of real world case studies that demonstrate the benefits of using traffic analysis tools for the project. Guidance Volume 5 Home Page Listing
Answer: This document provides transportation professionals with understanding of some of the most commonly used measures of effectiveness (MOEs) generated by traffic simulation and analytical tools. and provide guidance on the calculation of the MOEs in some of the common analysis tools. It is hoped that this document will assist the transportation community in creating a more consistent process in the interpretation of MOEs from various analysis tools. Guidance Volume 6 Home Page Listing
Answer: This document provides insights into the common pitfalls and challenges associated with use of traffic analysis tools for predicting future performance of a transportation facility. It provides five in-depth case studies that demonstrate common ways to ensure appropriate results when using an microsimulation tool, and also includes "how to" material that allows users to address common challenges associated with microsimulation analysis. Guidance Volume 7 Home Page Listing
Answer: This document provides guidance to decision-makers at agencies and jurisdictions considering the role of analytical tools in work zone planning and management. It is often unclear what kind of analytical approach may be of most value, particularly in light of complex data requirements and staff training. The decision to create an analytical capability to support decision-making can be a significant investment, and deserves careful consideration. Guidance Volume 8 Home Page Listing
Answer: This guide targets practitioners and includes more detailed real world and application information to help guide the selection of a modeling approach as well as specific project applications. This would be where tailoring the analysis begins, moving towards choosing what type of approach to administer and finally selecting the appropriate category of tool(s). This document uses examples/case studies/real world applications to help support the guidance provided. Guidance Volume 9 Home Page Listing
Answer: In the past, much, or all, of recurring congestion was felt to be a systemic problem ("not enough lanes") but much of the root cause of recurring congestion is in fact subordinate locations within a facility; i.e., "bottlenecks" and chokepoints. Elsewhere on the same facility and during the same hours, the facility runs free. This document is meant to discuss when, where and how to study small, localized sections of a facility (e.g., on/off ramps, merges, lane drops, intersections, weaves, etc.) In cost-effective means. Some chokepoints are (or seem) obvious in their solution; add a turn lane, widen a stretch of highway, retime a signal, or separate a movement by ramp. However, the solution can often lead to hidden or supplementary problems; hidden bottlenecks, disruptions upstream, or undue influence on abutting accesses, etc Analyzing localized sections of highway is different from analyzing entire corridors or regions. Micro simulation analysis products vary in their target applications and purported results. This document will provide guidance that specifies the choice of analysis tools and inputs necessary to analyze localized problem areas. It also provides some guidance as to when analysis it warranted, and what data inputs are required. Guidance Volume 10 Home Page Listing
Answer: This module was developed to guide traffic engineers and transportation operations managers in analyzing and modeling weather impacts on transportation system. It is expected that by utilizing the weather module, better estimates and predictions of real world traffic parameters during inclement weather conditions can be made, leading to appropriate measures for improving highway safety and mobility in inclement weather. Guidance Volume 11 Home Page Listing
Answer: Work Zone Traffic Analysis (WZTA) is the process of evaluating and determining the safety and mobility impacts within a construction, maintenance, or rehabilitation project. Establishing a procedure for analyzing work zone mobility and safety impacts aids agencies in the planning, decision-making, design, and financial aspects of the project. This report provides guidance on WZTA applications and presents the steps involved in the application of maintenance of traffic alternatives analysis (MOTAA) and decision framework. It contains step-by-step guidance to assist the analyst in determining the most suitable tools to perform the work zone analysis and presents a procedure for developing and applying models to work zone analysis. To illustrate the MOTAA process and decision framework contained in this guidebook, a variety of WZTA case studies are presented to demonstrate a diverse set of MOTAA applications. Guidance Volume 12 Home Page Listing