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.

• 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 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 expected difference between estimated 85th percentile speeds along the highway and the design speed of the highway, and
• The expected reduction in estimated 85th percentile speeds from an approach tangent to its succeeding horizontal curve.

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:

• The measures reflect nationwide average conditions on State-maintained, paved, two-lane rural highways with 55 mi/hr posted speed limits. The speed-profile model was calibrated using data from more than 200 sites on two-lane rural highways in six States. This database is relatively large and is considered nationally representative, but the resulting model should not be expected to reflect all sources of regional and highway-specific variations. Therefore, it is important to apply local knowledge in interpreting model results.
• There is no accepted "standard" for acceptable or unacceptable values of the two measures. The output is color-coded to help users distinguish larger values from smaller values. The color-coding was not intended and should not be interpreted as categorizing outputs as acceptable or unacceptable.

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.