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

IHSDM Intersection Diagnostic Review Model

APPENDIX A – IDRM TREATMENT DETAILS

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.

Appendix A is intended to provide additional information on recommended treatment options that are generated by executing IDRM.  The information provided in Appendix A is intended to provide a sampling of what types of information may ultimately be provided to the IDRM user.  Basic information is expected to be gathered from readily available sources such as AASHTO Policy, State DOT design manuals, the Highway Capacity Manual, NCHRP Research Reports, etc.

Information presented in this document is in text form. Eventually, information could be made available within the IHSDM framework to provide online help.  Real-time links to key sources of information should be considered as the concept is developed further.

Appendix A highlights information on treatment options for skewed intersections and intersections without turning lanes.  Discussion of these topics is not intended to be all-encompassing.  Rather, the information presented herein is intended to provide the reader with a sense of the type of information that is expected to be ultimately provided to IDRM users.

Skewed Intersection Treatment Details

Roadways intersecting at acute angles tend to:

  • Increase the time required for vehicles to cross the intersection.
  • Have large turning roadway areas (increased pavement area).
  • Limit the visibility of turning vehicles (sight distance problems).

Geometric design improvements generally focus on realigning, relocating, or closing one or more legs of the intersection to improve the angle of intersection.  The reader is directed to two sources highlighting implementation of treatment options:

  1. AASHTO Policy (pp. 584-585) - The information from these pages, which pertains to roadway alignment, will be displayed.
  2. Ohio Department of Transportation (ODOT) Location and Design Manual Volume 1: Roadway Design - The intersection at-grade information from p. 4-1 will be displayed.

(Note: ODOT design manual information can be substituted with any DOT standard practice.)

Turn-Lane Treatment Details

Turn lanes are desirable from both a safety and an operational standpoint. Providing a turn lane allows decelerating and stopped vehicles to remove themselves from the through-traffic stream and can increase the available capacity at an intersection.  Desirably, the auxiliary lane should allow for deceleration and storage of turning vehicles.

The reader is directed to two sources highlighting implementation of treatment options:

  1. AASHTO Policy (pp. 717-720) - The information from these pages will be displayed.
  2. Ohio Department of Transportation Location and Design Manual Volume 1: Roadway Design - The information pertaining to turn-lane treatments, pp. 4-3 and 4-4, and the accompanying figures from Sections 401-5 through 401-8, will be displayed.

(Note: ODOT design manual information can be substituted with any DOT standard practice.)

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