9.2 Sketch-Level or Macroscopic Analysis

9.2.1 Overview and Intended Use

Sketch-level or macroscopic analysis looks at deterministic relationships between traffic variables such as flow, speed, and density to evaluate segment operations (FHWA Traffic Analysis Toolbox Volume II: Decision Support Methodology for Selecting Traffic Analysis Tools). Deterministic means that for a set of inputs, there is only one answer. In this context, it means the characteristics of individual vehicles are not tracked on the system-wide level. Performance measures are aggregated for platoons of vehicles over specified time intervals. Network details are approximated or based on averages.
Sketch-level or macroscopic analysis is supported by observed patterns of traffic variables. Sketch-level or macroscopic analysis tools typically need the least amount of input and detail and are often used for projects in the planning stage of project development or for projects that do not need high levels of detail. Sketch-level or macroscopic analyses are typically used for analyzing a large network, or screening multiple alternatives. It is not well suited for analyzing unique transportation networks. If a network is considerably modified from the original demand or network attributes, obtain new OD tables before analyzing it.
The purpose of this section is to provide an overview of the tools available to complete a Sketch-level or macroscopic analysis and provide guidance on when to use the different tools, the data and input needed for each tool, and the MOEs used to evaluate operations.
TDM and service volume tables are the main tools used analyze segments at a Sketch-level or macroscopic level. The outputs of a TDM are typically used if the segment to analyze is available in the model. In some instances, TDM provides a more detailed analysis than is necessary for the project stage or scope, in which case the service volume tables can be used. Service volume tables are discussed in depth below.

9.2.2 Analysis Tools

Sketch-level or macroscopic analysis of segments can be performed with either TDM outputs or service volume tables. TDM outputs are typically used for all Sketch-level or macroscopic analysis, except for instances when a basic analysis is adequate. In this case, the service volume tables can be used.

9.2.2.1 Travel Demand Model

TDM is a macroscopic analysis tool that is best for analyzing a large area with low precision. There are different types of TDMs, which include four-step/trip-based models and ABM. The traditional four-step model includes trip generation, trip distribution, mode split, and traffic assignment for a single OD from traffic analysis zone to traffic analysis zone. Conversely, ABMs examine continuing trips from one zone to another over time.
TDMs are best used for subregional/subarea-, regional-, and state-level planning and serves as a support tool for long-range areawide transportation planning. TransCAD and Cube are common software programs used to complete this analysis (for detailed guidance on inputs and outputs for these software programs, see their respective user guides). Highway and corridor networks are coded into the software program, along with socioeconomic data for each traffic analysis zone. Facility type (e.g., major and minor arterials, collectors, and locals), number of lanes, and area type impacts both the intensity and usage of links within the model.
TDMs are typically maintained by the local MPO. TxDOT’s SAM can be used in rural areas where an MPO TDM is not available. TDM outputs are typically all that is used for an analysis, however, there may be circumstances for updating and running the TDM for detailed analysis depending project scope and on project impacts to the existing network. A TDM is typically updated, calibrated, and validated for large projects or when major data is updated (i.e., census data, land-use data). When an update is necessary, the basic steps to develop/update and calibrate a TDM are:
  • TDM Development and Update; and
    • Collect data (see data and input parameters section)
    • Update Traffic Analysis Zone (TAZ) structure using base year demographic and socioeconomic data
    • Use HCM-based (mesoscopic) procedures for roadway capacities and intersection delay
    • Verify coding of urban and rural area types
    • Follow NCHRP Report 716 Travel Demand Forecasting and FHWA’s Model Validation and Reasonableness Checking Manual
  • TDM Calibration
    • Use big data resources (StreetLight, INRIX, Replica, etc.) to develop, calibrate, validate, and refine the TDM. Reinforce big data with statewide travel survey data regarding trip purpose, occupancy, and multi-modes
    • If corridor extends beyond TDM limits, calibrate external stations with historical and collected data
TDM outputs include multimodal time-dependent OD tables, link volumes, vehicle miles traveled, turning movement percentages, and network performance. These outputs can be used in alternatives analysis, traffic projections, HCM-based, mesoscopic, microsimulation, and other forms of traffic analysis for segments. Traffic and Safety Analysis Procedures Manual | 2024 9-6 When there are discrepancies between actual counts and TDM outputs, actual count data is typically used.

9.2.2.2 Service Volume Tables

Generalized service volume tables are used for segment analysis, as described in . Service volume tables from the HCM 7th Edition can be used to evaluate individual segments or whole facilities of freeways, highways, and arterials. The roadway sections within a study area that are highest traveled should be selected for analysis.
A segment analysis using service volume tables following the below steps:
  • Collect necessary data for the segment type ();
  • Use reference tables to determine service volume for the facility in question () and collected data (as shown in ); and
  • An existing or anticipated volume higher than the service volume indicates the need for further analysis
If all the segments within a facility are operating within capacity (i.e., existing, or anticipated volumes are less than calculated service volume), there may not be queue buildup into other segments, indicating that a full facility analysis is not needed. These tables estimate maximum daily thresholds for a given LOS and default conditions. They are typically used as a general tool to identify specific segments that may need further evaluation. The tables are set for specific conditions and cannot be used as a final decision-making tool or as a substitute for engineering judgment. Comparing a segment’s demand to the service volume thresholds can provide a quick estimation of LOS and volume-to-capacity ratios of a segment.
Table 9-2: Service Volume Reference Tables
Segment Type
Reference Table
Freeways (Basic)
Appendix J, Section 3, Table 1 (urban) or Appendix J, Section 3, Table 2 (rural)
Multi-Lane Highways (Basic)
Appendix J, Section 3, Table 3 (urban) or Appendix J, Section 3, Table 4 (rural)
Two-Lane Highways
Appendix J, Section 3, Table 5
Arterial Facilities
Appendix J, Section 3, Table 5

9.2.3 Data and Input Parameters

9.2.3.1 Travel Demand Model

If updating the TDM is necessary, collect the following data to complete the analysis:
  • Socioeconomic and demographic data;
    • Latest census
    • Land use data
    • Travel surveys
  • OD data, such as Streetlight, Replica, and INRIX (also outputs travel times);
  • Traffic counts; and
  • Geometric data
    • Segment: length, number of lanes, lane assignments, area type, posted speed and/or free flow speed, highway type
For more guidance on data and input parameters for a TDM, coordinate with the TPP Division for the SAM and MPOs for their respective TDMs.
Data requirements for macroscopic analysis of segments using service volume tables are shown in . Existing or anticipated traffic volumes are used to compare values from the service volume tables to determine whether the facility needs further analysis.

9.2.4 Measures of Effectiveness (MOEs)

9.2.4.1 Travel Demand Model Outputs

For macroscopic analysis using TDM outputs, typical MOEs include VMT, VHT, vehicle hours of congestion delay, vehicle hours of traffic control delay, travel speed, volumes, and the v/c. LOS can be computed using the volume-tocapacity ratio. The thresholds for LOS vary by location and the agency’s expectation of congestion level. Refer to
Appendix R, Section 3 – Traffic Tool Outputs
 for a TDM output
example
.

9.2.4.2 Service Volume Tables

LOS can be determined using the generalized service volume tables. The LOS is based on volume and capacity thresholds.
Table 9-3: Data Requirements for Service Volume Tables
Segment Type
Data Required
Freeways (Basic)
  • Urban or Rural designation
  • K-factor
  • D-factor
  • Number of lanes (both directions)
  • Terrain Type (Level or Rolling)
  • Existing or anticipated traffic volumes
Multi-lane Highways (Basic)
  • Urban or Rural designation
  • K-factor
  • D-factor
  • Number of lanes (both directions)
  • Terrain Type (Level or Rolling)
  • Existing or anticipated traffic volumes
Arterial Facilities
  • Posted Speed
  • K-factor
  • D-factor
  • Number of lanes (both directions)
  • Existing or anticipated traffic volumes