1.  Introduction

The Policy Review Module (PRM) Engineer's Manual is one component of the documentation supporting the Interactive Highway Safety Design Model (IHSDM). This introductory section: (1) provides a brief overview of IHSDM, (2) summarizes the capabilities and intended uses of the PRM, and (3) states the purpose and outlines the organization of the manual.

1.1  Overview of IHSDM

IHSDM is a suite of software analysis tools for evaluating safety and operational effects of geometric design in the highway project development process. The scope of the current release of IHSDM is two-lane rural highways.

IHSDM is intended as a supplementary tool to augment the design process. This tool is designed and intended to predict the functionality of proposed or existing designs by applying chosen design guidelines and generalized data to predict performance of the design. This tool is NOT a substitute for engineering judgment and does not create a standard, guideline or prescriptive requirement that can be argued to create any standard of care upon a designer, highway agency or other governmental body or employee. The use of this tool for any purpose other than to aid a qualified design engineer in the review of a set of plans is beyond the designed scope of this tool and is not endorsed by the Federal Highway Administration (FHWA).

The suite of IHSDM tools includes the following evaluation modules. Each module of IHSDM evaluates an existing or proposed geometric design from a different perspective and estimates measures describing one aspect of the expected safety and operational performance of the design.

• Policy Review Module (PRM)- The Policy Review Module checks a design relative to the range of values for critical dimensions recommended in AASHTO design policy.
• Crash Prediction Module (CPM)- The Crash Prediction Module provides estimates of expected crash frequency and severity.
• Design Consistency Module (DCM)- The Design Consistency Module estimates expected operating speeds and measures of operating-speed consistency.
• Intersection Review Module (IRM)- The Intersection Review Module leads users through a systematic review of intersection design elements relative to their likely safety and operational performance.
• Traffic Analysis Module (TAM)- The Traffic Analysis Module estimates measures of traffic operations used in highway capacity and quality of service evaluations.

Intended users of IHSDM results are geometric design decision makers in the highway design process, including project managers, planners, designers, and reviewers. The Federal Highway Administration's Flexibility in Highway Design document (Publication No. FHWA-PD-97-062) explains the context within which these decision makers operate:

The measures of expected safety and operational performance estimated by IHSDM are intended as inputs to the decision making process. The value added by IHSDM is in providing quantitative estimates of effects that previously could be considered only in more general, qualitative terms. The advantage of these quantitative estimates is that, when used appropriately by knowledgeable decision makers, they permit more informed decision-making.

The following general cautions should be considered in using IHSDM:

• Measures of expected safety and operational performance from IHSDM are only a subset of the large number of inputs that must be considered in making design decisions.
• Estimates from IHSDM are expected values, in the statistical sense, i.e., they represent the estimated average performance over a long time period and among a large number of sites with similar characteristics. Actual performance may vary over time and among sites. The estimates from IHSDM should not substitute for, but rather should supplement and complement local knowledge.
• While derived from the best available data using the best available methods, both the available data and methods have limitations. The engineer's manuals for each module document limitations that should be understood to apply appropriately the resulting estimates.

1.2  Overview of Policy Review Module

The basic functionality of the PRM is to automate the process of checking geometric design elements against relevant design policy documents. The PRM includes two basic components for each check: (1) electronic versions of "policy values," i.e., tables of recommended ranges of values for critical dimensions extracted from relevant AASHTO policy documents, and (2) a process for comparing "road values," i.e., the geometry of the highway being evaluated, against those "policy values."

The PRM tables of policy values and automated processes are based upon the following American Association of State Highway and Transportation Officials (AASHTO) documents: (1) A Policy on Geometric Design of Highways and Streets (1990, 1994, 2001 and 2004 editions), (2) Roadside Design Guide (2002 edition), and (3) Guide for the Development of Bicycle Facilities (1999 edition). The goal is to be faithful to these AASHTO documents in replicating the recommended range of values and in making appropriate comparisons between road values and policy values. The usage of terminology is also consistent with AASHTO definitions.

Users have the option to edit the tables of AASHTO policy values to adapt them to a particular State highway agency's design manual. Users exercise this option through the IHSDM Administration Tool. Users may edit any or all of the values in any or all of the tables. The ability to edit policy tables has the following limitations: (1) users must operate within the set of existing tables; it is not possible to create additional tables, and (2) in editing a table, users are constrained by the number of columns and column headings, i.e., it is not possible to add columns or to change which data element the column defines. Additional details on using the IHSDM Administration Tool are found in the System Administration section of the Documentation.

Initial steps in running the PRM include specifying the type of project (new construction or reconstruction), the relevant policy and the geometry elements to be checked. The "Policy" pull down menu of the PRM Evaluation includes six standard options: AASHTO 1990 Policy (English), AASHTO 1994 Policy (Metric), AASHTO 2001 Policy (English), AASHTO 2001 Policy (Metric), AASHTO 2004 Policy (English), and AASHTO 2001 Policy (Metric). Additional policy options will appear if the user has edited and saved additional policies through the IHSDM Administration Tool.

The PRM begins each check by determining whether or not all design and control data necessary to complete the check are available. Before choosing the "Run" option at the end of the PRM Evaluation Wizard, users have the opportunity to review the list of data that may have some issues and the option of revising/adding the data. At the end of this process, PRM runs the selected checks with whatever data are available.

PRM results are provided in an Evaluation Report, which may be viewed in html browser. The results for each check are summarized in one or more tables. For two checks (Stopping Sight Distance and Passing Sight Distance), graphical output is also provided. Users also have options for on-screen display of the sight distance graphs through the "Show Graphs" button on the Evaluation Operations panel.

The intent of the PRM is the same as the intent of AASHTO policy documents, as stated in the Foreword to the 2001 edition of AASHTO's A Policy on Geometric Design of Highways and Streets, "to provide guidance to the designer by referencing a recommended range of values for critical dimensions." Caveats regarding intended and unintended uses of design policy also apply to the IHSDM PRM.

The relevance of the PRM is the same as the relevance of the policies upon which it is based. For example, the Foreword to AASHTO's 2001 A Policy on Geometric Design of Highways and Streets states it "is not intended as a policy for resurfacing, restoration, or rehabilitation (3R) projects." It is the user's responsibility to determine which policy is relevant to a particular design project and which elements of design should be checked. The IHSDM PRM evaluates user-specified elements against a user-specified policy. Therefore, output from the Module documenting a check of a given element against a given policy should not be interpreted as meaning the element needed to be checked or that the policy checked was the relevant policy.

PRM has potential application throughout the project development process. The following are examples of project development applications for the PRM:

• An evaluation of existing highways to support design investigations and alternatives development is a task performed early in reconstruction projects.
• Quality control/quality assurance is an obvious use of the PRM, particularly for project managers and review agencies when comparing preliminary engineering to final design.
• Documentation of alternatives to support environmental studies and internal agency needs also is a use of the PRM.
• Preparation of design exception requests and documentation of design exceptions.

In all of these applications, other IHSDM modules (e.g., Crash Prediction, Design Consistency, and Traffic Analysis) might also be used to estimate the expected operational and safety performance of the highway. These performance measures can be used as input to decision-making and in documenting the basis for decisions.

1.3  Purpose of This Manual

The purpose of the PRM Engineer's Manual is to provide sufficient information about the functionality and scope of the PRM that users can make appropriate judgments about whether and how to use it beneficially at any stage of a design project, and can make appropriate interpretations and applications of PRM outputs based upon an understanding of the PRM. The manual includes useful information about the data required to perform checks and describes the process used to perform the checks.

The manual is organized into the following sections:

• Section 2., Input Data Requirements
• Section 3., Summary of PRM Checks
• Section 4., Cross Section Checks
• Section 5., Horizontal Alignment Checks
• Section 6., Vertical Alignment Checks
• Section 7., Sight Distance Checks