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Publication Number: FHWA-RD-02-045
Date: March 2003

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.

Table 2.  Priority Issues Identified for Knowledge Base Development

1.      Intersection Configuration

  1. Multileg intersections
  2. Skewed intersections
  3. Offset T intersections
  4. More than one minor-road approach on the same side of the major road

2.       Horizontal Alignment

  1. Intersection on horizontal curve
  2. Curve on intersection leg
  3. Approach alignment differs between opposing approaches (lane continuity through intersection

3.       Vertical Alignment

  1. Intersection on crest vertical curve
  2. Crest vertical curve on intersection approach
  3. Steep grade through intersection
  4. Intersection on sag vertical curve
  5. Continuity of minor-road profile through intersection

4.       Warrants for Auxiliary Lanes

  1. Warranted left-turn lane not present
  2. Warranted right-turn lane not present

5.       Intersection Sight Distance (ISD)

  1. Lack of ISD in one or more quadrants

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:

Concern

Design element

Treatment

Treatment type

Insufficient stopping sight distance on leg entering from north

Crest vertical curve on intersection approach

Flatten vertical curve to provide adequate sight distance

Design improvement

Insufficient stopping sight distance on leg entering from north

Crest vertical curve on intersection approach

Install warning flashers on approach

Mitigation measure

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 3. IDRM Concerns

Section

Concerns related to the intersection as a whole

4.20

Insufficient left-turn bay storage length between closely spaced intersections

4.21

Insufficient taper length for left-turn lanes between closely spaced intersections

4.22

High traffic conflict index

4.24

Large intersection pavement area

 

Concerns related to individual intersection legs

4.1

Insufficient intersection sight distance (ISD)

4.2

Insufficient intersection sight distance (ISD) for intersection on horizontal curve

4.3

Insufficient intersection sight distance (ISD) for a horizontal curve on an intersection approach

4.4

Insufficient intersection sight distance (ISD) for intersection on approaches to crest vertical curve

4.5

Insufficient intersection sight distance (ISD) for multileg intersection

4.6

Insufficient intersection sight distance (ISD) for intersection with more than one minor-road approach on the same side of the major road

4.7

Insufficient intersection sight distance (ISD) for skewed intersection

4.8

Insufficient stopping sight distance (SSD) for a crest vertical curve on an intersection approach

4.9

Insufficient stopping sight distance (SSD) for a horizontal curve on an intersection approach

4.10

Insufficient visibility of traffic signal

4.11

Insufficient visibility of stop sign

4.12

Insufficient visibility of yield sign

4.13

Insufficient decision sight distance (DSD) for a crest vertical curve on an intersection approach

4.14

Insufficient decision sight distance (DSD) for a horizontal curve on an intersection approach

4.15

Insufficient decision sight distance (DSD) for multileg intersection

4.16

Insufficient decision sight distance (DSD) for intersection with more than one minor-road approach on the same side of the major road

4.17

Increased crossing distance

4.18

Warranted left-turn lane not present

4.19

Warranted right-turn lane not present

4.23

Uneven, discontinuous minor-road profile through intersection

4.25

Approach alignment differs between opposing approaches

4.26

Insufficient queue storage

4.27

Loss of control potential due to frequent braking

 

Table 4. IDRM Models

Section

Model

Intersection sight distance (ISD)-based models

3.1

Intersection Sight Distance for Case B1 – Left Turn From Minor Road

3.2

Intersection Sight Distance for Case B2 – Right Turn From Minor Road

3.3

Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road

3.4

Intersection Sight Distance for Case F – Left Turn From Major Road

Stopping sight distance (SSD)-based models

3.5

Stopping Sight Distance for Vertical Curves

3.6

Stopping Sight Distance for Horizontal Curves

3.7

Visibility Distance to Traffic Signal

3.8

Visibility Distance to Stop Sign

3.9

Visibility Distance to Yield Sign

Decision sight distance (DSD)-based models

3.10

Decision Sight Distance for Vertical Curves

3.11

Decision Sight Distance for Horizontal Curves

Clearance time models

3.12

Clearance Time for Skewed Intersection

Turn-lane models

3.13

Left-Turn Lane Warrants

3.14

Right-Turn Lane Warrants

3.15

Left-Turn Lane Length for Closely Spaced Intersection

Miscellaneous models

3.16

Intersection Conflict Index

3.17

Minor-Road Profile

3.18

Intersection Pavement Area

3.19

Change in Approach Alignment Between Opposing Approaches

3.20

Queue Length Prediction

3.21

Horizontal Curve Design for Braking and Cornering


Table 5.  Application of IDRM Models to Evaluate Concerns

Section

Concern

Model(s) Used to Evaluate Concern

Concerns related to the intersection as a whole

4.20

Insufficient Left-Turn Bay Storage and Deceleration Length Between Closely Spaced Intersections

3.15   Left-Turn Lane Length for Closely Spaced Intersection
3.20   Queue Length Prediction

4.21

 Insufficient Taper Length for Left-Turn Lanes Between Closely Spaced Intersections

3.15   Left-Turn Lane Length for Closely Spaced Intersection

4.22

High Traffic Conflict Index

3.16   Intersection Conflict Index

4.24

Large Intersection Pavement Area

3.18   Intersection Pavement Area

4.1

Insufficient Intersection Sight Distance

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.2

Insufficient Intersection Sight Distance for Intersection on a Horizontal Curve

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road       
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.3

Insufficient Intersection Sight Distance for a Horizontal Curve on an Intersection Approach

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.4

Insufficient Intersection Sight Distance for Intersection on Approaches to Crest Vertical Curve

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.5

Insufficient Intersection Sight Distance for Multileg Intersection

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.6

Insufficient Intersection Sight Distance for Intersections With More Than One Minor-Road Approach on the Same Side of the Major Road

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.7

Insufficient Intersection Sight Distance for Skewed Intersections

3.1     Intersection Sight Distance for Case B1 – Left Turn From Minor Road
3.2     Intersection Sight Distance for Case B2 – Right Turn From Minor Road
3.3     Intersection Sight Distance for Case B3 – Crossing Maneuver From Minor Road
3.4     Intersection Sight Distance for Case F – Left Turn From Major Road

4.8

Insufficient Stopping Sight Distance

3.5     Stopping Sight Distance for Vertical Curves

4.9

Insufficient Stopping Sight Distance for a Horizontal Curve on an Intersection Approach

3.6   Stopping Sight Distance for Horizontal Curves

4.10

Insufficient Visibility of Traffic Signal

3.7     Visibility Distance to Traffic Signal

4.11

Insufficient Visibility of Stop Sign

3.8     Visibility Distance to Stop Sign

4.12

Insufficient Visibility of Yield Sign

3.9     Visibility Distance to Yield Sign

4.13

Insufficient Decision Sight Distance for a Crest Vertical Curve on an Intersection Approach

3.10   Decision Sight Distance for Vertical Curves

4.14

Insufficient Decision Sight Distance for a Horizontal Curve on an Intersection Approach

3.11   Decision Sight Distance for Horizontal Curves

4.15

Insufficient Decision Sight Distance for Multileg Intersection

3.10   Decision Sight Distance for Vertical Curves
3.11   Decision Sight Distance for Horizontal Curves

4.16

Insufficient Decision Sight Distance for Intersections With More Than One Minor-Road Approach on the Same Side of the Major Road

3.10   Decision Sight Distance for Vertical Curves
3.11   Decision Sight Distance for Horizontal Curves

4.17

Increased Crossing Distance

3.12   Clearance Time for Skewed Intersection

4.18

Warranted Left-Turn Lane Is Not Present

3.13   Left-Turn Lane Warrants

4.19

Warranted Right-Turn Lane Is Not Present

3.14   Right-Turn Lane Warrants

4.23

Uneven, Discontinuous Minor-Road Profile Through Intersection

3.17   Minor-Road Profile

4.25

Approach Alignment Differs Between Opposing Approaches

3.19   Change in Approach Alignment Between Opposing Approaches

4.26

Insufficient Queue Storage

3.20   Queue Length Prediction

4.27

Loss of Control Potential Due to Frequent Braking

3.21  Horizontal Curve Design for Braking and Cornering

 

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