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1.  Introduction

This 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 Design Consistency Module, and (3) states the purpose of this particular 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.

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:

An important concept in highway design is that every project is unique. The setting and character of the area, the values of the community, the needs of the highway users, and the challenges and opportunities are unique factors that designers must consider in each highway project. Whether the design to be developed is for a modest safety improvement or 10 miles of new-location rural freeway, there are no patented solutions. For each potential project, designers are faced with the task of balancing the need for the improvement with the need to safely integrate the design into the surrounding natural and human environment.

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:

1.2  Overview of Design Consistency Module

Design consistency refers to a design's conformance with drivers' expectations. It is a goal of design. In general terms, and with all other factors being constant, one would expect crash frequency to decrease as design consistency increases.

One of drivers' expectations on rural two-lane rural highway alignments is to be able to maintain a relatively uniform speed; complete uniformity (e.g., being able to set one's cruise control and never having to adjust it), however, is neither common nor, therefore, expected. While it has been the subject of research for several decades, commonly accepted procedures for estimating operating speeds and evaluating speed consistency have been lacking. The Design Consistency Module represents a significant milestone in efforts to produce such a procedure.

The Design Consistency Module evaluates operating speed consistency through a speed-profile model that estimates expected 85th percentile, free-flow, passenger vehicle speeds along a highway. The speed-profile model combines estimated 85th percentile speeds on curves (horizontal, vertical, and horizontal-vertical combinations), desired speeds on long tangents, acceleration and deceleration rates exiting and entering curves, and an algorithm for estimating speeds on vertical grades.

The module estimates two measures:

The intended use of these measures is to highlight locations where additional attention and evaluation may be warranted.

With respect to the first measure, it is not uncommon and not necessarily a problem for estimated 85th percentile speeds to be greater than the design speed of the highway. It is not necessarily a problem because, in part, design policy and practice incorporate considerable margins for safety in design parameters based upon design speed. On many sections of highway, drivers can and, therefore, do comfortably operate at speeds higher than the highway's design speed. The additional evaluations that may be warranted include, for example, checking Policy Review Module output for available sight distances near minimum recommended policy values, and checking actual crash history for speed-related crash patterns.

The second measure estimates the expected speed reduction from an approach tangent to a horizontal curve. Locations where the expected speed reduction is large may warrant additional evaluations; for example, whether adjustments to the alignment would be cost effective. Often, physical constraints dictate and cannot be overcome. Where large expected speed reductions cannot be reduced, then consideration should be given to warning drivers and possible accommodations through cross section and roadside design.

While design consistency has long been considered in general terms, the quantitative measures provided by the Design Consistency Module have not been routinely available to project decision makers. Some general cautions that should be exercised in using these measures include:

1.3  Purpose of This Manual

The Design Consistency Module Engineer's Manual documents the basic information that users should understand in order to make appropriate use of the Module. It details the data input requirements, explains the procedural elements of the module, enumerates the steps in the design consistency algorithm, and describes the presentation of model outputs. Throughout, this manual highlights limitations of the Module that users should consider in applying it and interpreting results.

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