4.4 Analysis Tool Evaluation
Project-specific constraints may merit the selection of analysis tools different from tools prescribed per the Analysis Tool Selection Process provided
in Appendix E, Section 2 – Traffic Analysis Tool Selection Process
. Tool limitations are reviewed when considering project constraints such as project complexity, schedule, and budget. Analysis tool limitations, provided in , are described in terms of complexity and data requirements; each criterion is classified as either low, medium, or high. Analysis tool method is described as deterministic (reports the same results given the same inputs) or stochastic (reports different results given the same inputs due to the inclusion of randomizing factors). See the Section 4.4.2
subsection for more information on deterministic and stochastic analysis types. Analysis time is presented as a comparative factor. Bigger time factors indicate longer time for analysis, while smaller factors indicate shorter times.Analysis Tool | Method | Time | Complexity | Data Requirements |
|---|---|---|---|---|
Spreadsheet-based Tools | Deterministic | Varies | Varies | Varies |
TDM | Deterministic | 2x | Low to Medium | Medium |
CAP-X | Deterministic | 1x | Low | Medium |
HCS | Deterministic | 2x | Low to Medium | Medium |
DynusT | Deterministic/ Stochastic | 4x | Medium to High | Medium to High |
Synchro | Deterministic | 2x | Medium | Medium |
SIDRA | Deterministic | 3x | Medium | Medium |
Vissim | Stochastic | 5x | High | High |
CORSIM | Stochastic | 4x | High | High |
SimTraffic | Stochastic | 3x | Medium to High | Medium |
4.4.1 General
Analysis results are the most reliable when typical geometric configurations are analyzed. All analysis tools have some limitations. Below are some examples of the limitations of deterministic tools:
- Network affect caused by congestion;
- Traffic flows will not adjust travel routes due to congestion;
- Interference caused by roadside parking;
- HOV, High-Occupancy Toll (HOT), or other managed lanes;
- Tight diamond interchanges;
- Impacts of multiple closely spaced driveways along a street segment;
- Interactions between bicycles, pedestrians, and vehicles sharing the same roadway;
- Impacts of on-street parking, commercial vehicle loading, and double parking;
- Impacts of changes in roadway environment (or other violations of driver expectation);
- Impacts of tolling on travel patterns;
- Driver behavior;
- Two-way left-turn lanes; and
- v/c does not capture spatial congestion
The following are some examples of the limitations of stochastic tools:
- Models may have difficulty capturing the effects of a continuous two-way left-turn lane;
- It is recommended to run models multiple times to capture randomness or average travel experience; and 4-11 2024 | Traffic and Safety Analysis Procedures Manual
- Impacts of driver behavior (obstruction of visibility, roadway distractions, etc.)
4.4.2 Method
Analysis tools may be classified as either deterministic or stochastic. Stochastic tools account for randomness, while deterministic tools do not. Because of this, stochastic tools more accurately represent variability present in a traffic network (e.g., varying speeds among drivers, varying reaction times, and varying acceleration and deceleration rates). Microsimulation-based (microscopic) tools are typically stochastic, while the HCM-based and spreadsheet-based tools are deterministic.
4.4.3 Level of Effort
Level of effort refers to the duration necessary for model development and application. Level of effort is critical to understand for all projects and particularly for projects with set letting schedules. If constrained by time limits, PS&E projects may select Planning-stage or Preliminary Schematics-stage analysis tools. FHWA’s
Guidance on the Stage of Effort Required to Conduct Traffic Analysis Using Microsimulation
presents estimates of typical microsimulation (microscopic) traffic analysis projects based on the project size. shows estimated level of effort in terms of a range of percentages by task for microsimulation (microscopic) projects based on the FHWA guidance. For small- and medium-sized projects, level of effort is similar across different tasks, as the modeling complexity is still comparable. For large models, calibration efforts need extensive data collection. Although it could seem that effort is lower for developing and calibrating the base model and performing alternatives analysis, the small difference in percentages could represent hundreds of hours of effort. For example, a small project that has a duration of six months will need approximately 15 days of data collection efforts.
Task | Project Size | ||
|---|---|---|---|
Small | Medium | Large | |
Data Collection | 5-10% | 5-10% | 15-20% |
Base Model Development and Calibration | 25-30% | 30-35% | 25-30% |
Alternatives Analysis | 50-55% | 45-50% | 25-30% |
Documentation/Presentations | 5-10% | 5-10% | 15-20% |
Project Management | 5-10% | 5-10% | 10-15% |
4.4.4 Complexity
Complexity indicates the level of effort necessary to develop, calibrate, and validate a model for it to achieve its intended purpose. Complexity varies by project type, project size, and the analysis tool, among other things. Due to the variability in project complexity, it is suggested that the analyst give careful consideration to selecting the correct tool for the project.
4.4.5 Data Requirements
Data requirements vary by project type, analysis tool, and selected MOEs. Review
Chapter 2
for detailed information on data requirements. FHWA Traffic Analysis Toolbox, Volume III (2004) guidance may be followed for TxDOT projects that do not involve FHWA approval. Average day traffic analysis may be performed for these projects. Follow FHWA Traffic Analysis Toolbox, Volume III (2019) guidance for projects that involve IAJRs or FHWA approval.