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Interactive Highway Safety Design Model (IHSDM): Policy Review Module


The image shows the cover of the Design Manual and the cover of A Policy on Geometric Design of Highways and Streets. The Design Manual cover is yellow with a green stripe running horizontally at the bottom. The cover of A Policy on Geometric Design of Highways and Streets shows a city skyline at the top. Below that image are five smaller pictures detailing taxi cabs on a busy street, a highway overpass, an freeway intersection, pedestrians on a sidewalk, and a curved two-lane road.The Policy Review Module checks roadway-segment design elements for compliance with relevant highway geometric design policies. The module provides electronic files replicating quantitative policy values specified by AASHTO in the 1990, 1994, 2001, 2004, and 2011 editions of A Policy on Geometric Design of Highways and Streets and automates checks of design values against those policy values. The Interactive Highway Safety Design Model (IHSDM) also provides a tool for inputting policy tables from other agencies’ design policies.

The module organizes checks into four categories: cross section, horizontal alignment, vertical alignment, and sight distance. Cross-section checks include through-traveled way width and cross slope, auxiliary lane width and cross slope, shoulder width and cross slope, cross slope rollover on curves, and bridge width. Horizontal alignment checks include radius of curvature, superelevation rate, length of horizontal curve, and compound curve ratio. Vertical alignment checks include tangent grade and vertical curve length. The Policy Review Module also can check stopping, passing, and decision sight distance.

Additional checks planned include: clear zone, roadside slope, normal ditch design, and superelevation transition.

This module can be applied at several stages in the highway design process. During project planning and scoping for improvement projects on existing roadways, it can provide an initial assessment of how the existing geometric design compares to current design guidelines. For all projects, it can facilitate quality assurance checks throughout detailed design and design review.