Making two-lane roads safer: IHSDM is an invaluable safety evaluation software package for highway designers and planners

Public Roads, Jan-Feb, 2003 by Raymond A. Krammes, Carl Hayden

Dewayne Sykes, assistant State roadway design engineer with the North Carolina DOT, observes, "The number of crashes is an indicator of safety easily understood by the nontransportation engineer. A tool that can provide this information will be invaluable when evaluating proposed highway designs and when explaining the relative safety of these designs to the public as well as others."

Design Consistency Module

The design consistency module help diagnose safety concerns at horizontal curves. Crashes on two-lane rural highways are overrepresented at horizontal curves, and speed inconsistencies are a common contributing factor to crashes on curves. This module provides estimates of the magnitude of potential speed inconsistencies.

The design consistency module uses a speed-profile model that estimates 85th percentile, free-flow, passenger vehicle speeds at each point along a roadway. (See "Managing Speed" on page 48.) 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 model was calibrated using speed data collected at horizontal curves and their approach tangents in six States. The module identifies two potential consistency issues: (1) large differences between the assumed design speed and estimated 85th percentile speed, and (2) large changes in 85th percentile speeds from an approach tangent to a horizontal curve.

Design consistency evaluations provide valuable information for diagnosing potential safety issues on existing highways. They also provide quality assurance checks of proposed preliminary and final alignment designs.

David Olson of the Washington State DOT explains, "The ability to evaluate design consistency from a perspective of driver expectancy is a tremendous addition to our project development toolset. Inattentive application of design guide lines presents opportunities for designs that conform to design policies and yet surprise the driver. The design consistency module is specifically developed to identify surprises and bring them to the attention of the designer. Once these locations have been identified, the designer can make modifications and quickly reevaluate their impacts."

Ron Erickson of the Minnesota DOT adds, "This is my favorite module because, as a designer, evaluating a project for design consistency is really difficult and for that reason rarely, done. Now the design consistency module will add another quality check into our design process."

Intersection Review Module

The intersection review module includes both policy and diagnostic review capabilities. The policy review component checks the following intersection design elements for compliance with design policy: corner radius, turn lane design, intersection angle, and intersection sight distance triangles.

The diagnostic review component system is an expert system that leads the user through a systematic evaluation of an existing or proposed intersection geometric design to identify potential safety concerns and possible treatments to address those concerns. The review considers design issues including: