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IHSDM Intersection Diagnostic Review Model

2.0     KNOWLEDGE BASE SCOPE AND APPROACH

2.1       Issues Addressed

A substantial effort was devoted to identifying the concerns addressed in the knowledge base.  The design team identified 111 potential design problems that could be considered for inclusion.  Given this large number of concerns and the multitude of possible combinations of them that may be found in intersection designs, priorities were established.  The highest priority issues, which the research team recommended that the knowledge base address, are listed in Table 2.

The research team concluded that the IDRM knowledge base should be more than a set of expert judgments for each of the above issues (and their permutations).  Study of the specific reasons why each issue could result in a design problem led to engineering models that can be used to evaluate the extent to which a problem is present.  These models are based on fundamental geometric design elements such as stopping sight distance (SSD), decision sight distance (DSD), intersection sight distance (ISD), and horizontal curve design.  Importantly, because they provide quantitative performance measures, the models allow evaluation not only of individual potential problems, but also of combinations of problems.  The models are summarized in Section 2.5.

2.2       Guiding Principles Used in Knowledge Base Development

A number of guiding principles were established to help in formulating models and identifying threshold values.  These principles identify the types of models and thresholds that are used to trigger concerns and the manner in which parameter values for the models are determined.

1. Maximum use is made of existing models used in geometric design, such as the models for stopping sight distance (SSD), intersection sight distance (ISD), and decision sight distance (DSD).  However, these models may be used with different parameter values than are used in established design policies.  The use of model parameters that differ from those used in design policies reflects the research team's judgment of good practice in situations different from the standard application of design policy.
2. Traffic volumes should have a role in defining the criticality of concerns.  An issue that warrants a Level 2 advisory at a low-volume intersection might well warrant a Level 1 advisory at a high-volume intersection.
3. Whenever possible, the existence and criticality of a problem should be determined from a model that accounts for the site-specific geometrics, traffic volumes, and speeds at and on the approaches to the intersection in question.
4. In general, models used to identify problems in IDRM should be based on the actual or estimated 85th percentile speed of vehicles on an intersection approach, rather than on the design speed that is more traditionally used in geometric design policies.  This approach is recommended because the actual 85th percentile speed should provide a more realistic representation of whether a safety problem exists than the design speed shown on the plans, which may be arbitrary.  Where field data on actual 85th percentile speeds are not available, predicted values, such as those recently developed by the Texas Transportation Institute in an FHWA study of the design consistency of horizontal and vertical alignment on two-lane highways, may be used.
5. In all models that use a speed variable, speed should be considered separately by the direction of travel.  Speeds may differ by the direction of travel because of horizontal alignment, vertical alignment, or cross-section factors.
6. The object heights used in sight distance models should be appropriate to the situation being analyzed and the specific target to be seen in that situation.  For example, the basic SSD model may be applied to several different issues with different object heights appropriate to each issue.  If the target to be seen is an oncoming vehicle traveling in the opposite direction on the same roadway, the appropriate object height is driver eye height.  If the target to be seen is a vehicle traveling in the same direction on the same roadway, the appropriate object height is taillight height.  If the target to be seen is a vehicle entering the roadway from a side road, the appropriate object height may be headlight height.
7. Only geometric features within a defined influence area of the intersection are considered.  The influence area extends 400 m from the center of the intersection along each approach and at least 200 m upstream from the beginning of any channelization or taper on the approach.  The 400-m influence area distance was selected to exceed the maximum DSD for any intersection approach speed.  Where geometric data for the entire intersection influence area are not available, IDRM will still operate with whatever data are available, but a significant portion of IDRM's diagnostic ability may be lost.
8. A passenger car is assumed to be the IDRM analysis vehicle with the exception of the Intersection Pavement Area model.  The models have been written in such a way that they are suitable to be expanded or modified in the future.

2.3       Highway Design Data

IDRM uses the detailed highway representation model provided by IHSDM.  This model provides a detailed representation of horizontal and vertical alignment, lane widths, traffic control, etc.  Data can be extracted from CAD files or input through the "IHSDM Edit/View Highway Information" interface.

Data from the IHSDM roadway model is generally sufficient for the application of IDRM.  If additional information is needed, the system requests it from the user.  One important example is the dimensions of clear sight triangles available at intersections.  While the design file can provide information on vertical geometry from which sight obstructions can be identified, it cannot be expected to allow identification of every possible obstruction (e.g., signs, seasonal vegetation) that a designer who has viewed the area would identify.  Therefore, in evaluating potential concerns requiring determination of clear sight triangles, IDRM always queries the user to confirm the available clear sight triangle unless the presence of a concern has already been established based on design file information alone.

2.4       Concerns Identified by IDRM

From the priority issues listed above in Table 2 and their combinations, 27 concerns or potential problems were identified to be addressed by the IDRM knowledge base.  These concerns are summarized in Table 3 .  It is important to note that the knowledge base is designed so that additional concerns, and the rules and models to evaluate them, can be added at any time.

2.5       Overview of IDRM Models

To address the concerns generated by the priority issues listed in Table 2 , 21 quantitative models were developed for the IDRM knowledge base.  These models are summarized in Table 4 . Section 3.0 provides a detailed description of each model.

2.6       Use of IDRM Models to Evaluate Concerns

Table 5 lists the IDRM models used to evaluate each of the 27 concerns.  Section 4.0 provides a detailed discussion of each concern, including a description of how the model(s) is used to evaluate the concern.

2.7       Overview of IDRM Treatment Recommendations

When IDRM identifies a potential intersection design concern, it also identifies design changes and other measures that could eliminate or mitigate the concern.  These are referred to as "treatments."  Treatments that could mitigate each identified concern are presented to the user through the IDRM user interface.

The IDRM knowledge base relates concerns to the applicable treatments by means of a matrix.  A row in this matrix is specified by a concern, a specific geometric condition or other design element related to the concern, and a treatment that addresses that condition.  Each treatment is categorized as either a "design improvement" (generally higher cost) or a "mitigation measure" (generally lower cost). For example:

In addition, for each treatment, IDRM provides Application Notes that describe the treatment in detail and present design guidelines for using the treatment.

The complete concern/treatment matrix is presented in Section 6.0.

Table 4. IDRM Models

Table 5.  Application of IDRM Models to Evaluate Concerns