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Publication Number: FHWA-HRT-11-064
Date: November 2011

 

Guide on The Consistent Application of Traffic Analysis Tools and Methods

CHAPTER 2. CHALLENGES TO CONSISTENT ANALYSIS AND RESULTS

This chapter describes the benefits of performing consistent analyses over the course of the project development life cycle and identifies the many challenges to achieving and maintaining consistent results throughout the various stages of project development.

First, the major stages of a typical project development life cycle are described. Current project development traffic analysis practice is described, followed by a brief summary of the major types of traffic analysis tools used in project development. The purpose of these early sections is to establish terminology. It is not the intent of this chapter to identify a specific project development process that all agencies must follow nor to identify the traffic analysis tools that all agencies must use.

The remaining sections of this chapter describe the benefits of maintaining a consistent analysis process over the course of project development and highlight the major challenges to achieving this objective.

THE PROJECT DEVELOPMENT LIFE CYCLE

A transportation improvement project goes through an extensive project development life cycle, starting with identification of a project need and ending with the construction of the project. During this time, the project traffic analysis may pass through many hands and many subdivisions of the organization. The major milestones of project development are: identification of project need, project initiation, project clearance, project PS&E, and, finally, project construction (see figure 2). For an example of the project development life cycle for a State transportation agency, see the California Department of Transportation's Project Development Procedures Manual.(2)

Chart shows the six major stages of the project development life cycle. In the project need stage, the steps are form team, conceptual design, environmental constraints, and local agency input. In the project initiation stage, the steps are project purpose, alternatives analysis, planning estimates, and preliminary environmental evaluation. In the project clearance stage, the steps are engineering studies, environmental documents, public hearings, and project approval document. In the plans, specifications, and estimates (PS&E) stage, the steps are plans, specifications, estimates, traffic plans, and work zone manage plan. In the construction stage, the steps are inspection and modifications. In the operation stage, the only step is monitoring.

Figure 2. Chart. The project development life cycle.

Project Need

The identification of project need is usually the result of one or more studies or plans. For cases where the project need statement is vague, a good analysis plan can help focus the need statement. At this stage of the project development process, the agency reviews the planning studies, reports, and plans that identified the need for the project. The agency identifies potential environmental constraints and defines the problem being addressed by the project. The agency establishes a project development team, prepares a traffic concept report, develops concept geometrics for the project, and obtains public and local agency input. At this point, the project need is well-defined, but the project itself is not yet defined in any great detail.

Project Initiation

At this stage, the agency prepares a formal project initiation document depending on the specific policies and procedures of the agency. This document usually defines the need and project purpose, develops concept alternatives, presents a value engineering analysis of project alternatives, identifies potential design exceptions, presents planning estimates, and provides a preliminary environmental evaluation. The product of this stage is the project initiation document.(2) Higher-level details of the project start to emerge at this step.

Project Clearance

At this stage, the project clearance documents are prepared. Engineering studies, project control surveys, environmental scoping, and environmental studies are performed. Public hearings are held on the environmental documents, responses to comments are prepared, and a preferred project alternative is selected. The products of this stage are typically the draft and final project approval report and the draft and final environmental document.

PS&E

Project PS&E are prepared at this stage, including the traffic plans and work zone traffic management plan to be employed during construction. All traffic analysis is complete at this stage.

Construction

The final stage of the project development process is the contracting and construction of the project. Supplemental traffic analysis may be performed at this stage to address issues that arise during construction. The agency's inspectors ensure that the traffic plans and construction traffic management plan are correctly implemented by the contractor.

Operation

Although not strictly part of the project development process, monitoring of the project's operation after it opens determines if the project operates as predicted by the traffic analyses conducted during the project development process. Lessons learned from monitoring project operation can be used to improve the traffic analyses performed during the project development process.

Each stage of the project development process requires different types of traffic analyses. At the early stages of the process, less is known about the project design and its likely opening date. As the various stages of the process are completed, more becomes known about the project design and its opening date. Thus, at the start, the traffic analysis must be more general, consider more alternatives, and employ more assumptions than at the later stages of project development (see figure 3).

Chart shows how the amount of information available for the project increases throughout the development life cycle. It shows how the required extent of assumptions decreases as project information increases.

Figure 3. Chart. Information versus assumptions during project life cycle.

CURRENT PRACTICE

The project development process varies considerably among agencies in the United States, but each stage of the project development life cycle imposes the following general requirements for traffic analysis and the tools used to perform those analyses.

Project Need

Project need is usually determined by one or more planning studies. At this stage, traffic analysis tools best suited to planning (transportation planning models and sketch planning) are applied. The traffic analysis is focused more on predicting demands and capacities than on predicting precise operational conditions. The tools employed at this stage must be able to consider long-term development trends (10 to 30 years in the future), multimodal transportation, and highway capacity improvements over a very large network. The objective is to obtain an approximate size of the facility needs to its nearest number of lanes.

Project Initiation

At the start of this stage, the long-term need for a transit or highway capacity improvement and the approximate geographic location of the need have already been identified. The project initiation stage seeks to better define the project through conceptual design and alternatives analyses. The traffic analysis is focused on further defining the project need and the range of solutions available for addressing the need. The traffic analysis tools used at this stage still must be able to deal with a relatively incomplete project description and must be able to rapidly evaluate several conceptual alternatives. At this stage, the tool must provide a slightly more precise estimate of traffic performance but need not consider subtleties of the specific design. The traffic analysis tool at this stage generally assumes the facility is designed and operated according to agency standards. Design exceptions and specific operation strategies (such as ramp metering or signal optimization) are generally not evaluated until the next stage of project development.

At this stage, sketch planning and travel demand models are often used. Deterministic/analytic HCM-based tools may be used.

Project Clearance

At the project clearance stage, all design and operation details of the proposed project are known. The traffic analysis tool must be able to take into account the project's design and operation specifics. In addition, since the final environmental analysis is performed at this stage, the tool must be able to evaluate the traffic performance characteristics and the effects of alternatives to the project on the project itself and other affected facilities in the area to a degree equal to that of the project itself. The traffic analysis must be sufficiently accurate and geographically/temporally comprehensive to support air quality and noise analyses for the project. The traffic analysis must also include the effects of the project and its alternatives on the demand for the facility and other affected facilities. The project clearance stage is the most analytically intensive stage of the project development process.

At this stage, the full spectrum of tool types may be used: sketch-planning tools, travel demand models, analytical/deterministic HCM-based tools, optimization tools, and simulation models.

PS&E

The preparation of PS&E for a project generally requires analyses to support the preparation of traffic plans for the project and the preparation of traffic management plans during construction. Traffic management analyses may require off-facility analyses of adjacent streets if construction detours require the use of off-facility routes.

The primary tools used at this stage are analytical/deterministic HCM-based tools. Optimization tools and simulation models may also be used.

Construction

During construction, rapid-response traffic analyses may be required to identify solutions to unanticipated traffic handling issues that arise during construction. The primary tools used at this stage are analytical/deterministic HCM-based tools. Optimization tools and simulation models may also be used.

Operation

The operating agency employs traffic analysis tools to identify the appropriate control settings to apply to the facility traffic controls (e.g., ramp meters and traffic signals). The tools used at this stage are typically optimization tools designed to find the best control parameters for a given situation. The analyses must be accurate and precise to the conditions observed in the field and often require further fine-tuning in the field before the final control settings are put in place.

At this stage, optimization tools are primarily used. Analytical/deterministic HCM-based tools and simulation models may also be used.

The operation stage is a good opportunity for the agency to evaluate the accuracy of its forecasts of the project operation. A monitoring program for recently opened projects can provide valuable feedback for future project development analyses.

THE TRAFFIC ANALYSIS TOOLBOX

Figure 4 shows the general focus of each tool category. Sketch planning and travel demand models are focused on predicting major changes of demand at the system level. Travel demand models can also produce facility-specific results, but they are less accurate for that purpose. HCM tools and traffic control optimization models do not predict changes in demand. HCM tools are best suited for specific bottleneck analyses but have been extended to facility analysis freeways and urban arterials. Optimization models are generally focused on single-facility analyses. Simulation models cover a wide range of geographic levels and dwell with bottlenecks and facilities. They can also be used for system analyses, but practical considerations (coding and computation requirements) generally limit the size of the systems they can cover. Simulation models may do some limited demand forecasting (primarily route choice).

Chart shows how the various tool types compare in terms of their ability to predict demand effects and in their geographic focus. Sketch planning tools have a system focus and show major demand effects. Travel demand models have a system focus and a facility focus and show major demand effects. Simulation models have a bottleneck focus, a facility focus, and a system focus and are midway between major demand effects and no demand effects. Deterministic Highway Capacity Manual (HCM) tools have a bottleneck focus and a facility focus and show no demand effects. Optimization tools have a facility focus and show no demand effects.

Figure 4. Chart. Geographic and demand focus of analysis tool categories.

Figure 5 characterizes the general resource requirements of each tool category and the degree of precision each tool delivers in terms of traffic performance predictions. Note that demand models focus their resources on demand modeling, rather than on traffic operations analyses. Thus, demand models typically require more resources than an HCM analysis but deliver poorer precision in the predicted traffic performance.

Chart shows how the various tool types compare in terms of the required resources and the precision of their traffic performance results. Simulation produces the greatest precision and requires the greatest resources. Sketch planning produces relatively limited precision with minimal resource requirements.

Figure 5. Chart. Resource requirements and precision of analysis tool categories.

The following descriptions of the major traffic analysis tool categories are adapted and condensed from Traffic Analysis Tools Program, Volume I: Traffic Analysis Tools Primer:(1)

Challenges to Consistent Analysis of Transportation Projects

The major challenges to the consistent analysis of transportation projects during their development life cycle are as follows:

The proper application of each traffic analysis tool type is a challenge in its own right. Major causes of erroneous results include the following:(1)

Why is Consistency Important?

Consistency in traffic analysis results throughout the project development life cycle is fundamental to quality. Throughout this document it is maintained that consistency flows from a focus on quality.

Obtaining consistent analysis results from project planning through project design ensures there are no surprises to undermine project delivery. Consistency offers benefits in terms of cost and time savings for the project delivery process as each succeeding stage of project delivery builds on the technical analysis from the previous stage. Naturally, the pursuit of consistent traffic analysis is an important means to achieve overall agency quality, enhancing the agency's credibility and effectiveness.

The main benefits of consistent analysis throughout the project development life cycle are as follows:

What Can Be Achieved?

While consistency is a commendable goal and a major component of an agency's effort to achieve quality of technical analyses, there is a limit to how much consistency can and should be achieved over the course of a project development life cycle.

When an agency starts the project development cycle, relatively little is known or defined for the project. Indeed, one of the major purposes of the project development process is to define those initially unknown details for the project. Thus, even if the agency were somehow able to employ the same tool and assumptions throughout the project development process, there would still be changes to those assumptions as the agency decides on the details of the project.

Secondly, the project development process is not instantaneous. The agency is unable to freeze its own work or that of other agencies in the area while it goes through the project development process. Thus, as other agencies develop new socioeconomic growth forecasts and make major decisions about projects, the agency will find that the assumptions it started with will rarely be current when the agency reaches the project clearance stage. Indeed, for some projects, changes in socioeconomic growth assumptions may occur over the course of the project clearance stage itself.

Thus, the goal of achieving and maintaining consistency throughout the project development process cannot be to maintain the same precise numerical results throughout the project development process. Instead, the goal should be to ensure that the initial decision to proceed with the project (as the details of the project are further refined during the process) is still the correct choice at the end of the process.

The goal of achieving consistent traffic analysis results in the project development process is to identify and adopt assumptions and traffic analysis procedures that are robust in the face of refinements to the project design and changes in rest of the world while the development process proceeds. If the project initiation stage is performed correctly, then the decision to proceed with project development should not be contradicted by the traffic analysis results at the project clearance stage.

An important feedback to the agency on the quality of its project development process is the operation of the project once it is opened. The performance of the facility should be as predicted during the latter stages of the project development process. If there are surprises when the project opens, then the agency needs to reconsider the tools and assumptions employed during the project development process.

 

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