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Interactive Highway Safety Design Model (IHSDM): Design Consistency Module


The image shows a two-lane rural highway in a forest. The highway curves slightly toward the left and there are vehicles on the right side of the road, driving away from the point of view of the photograph.

The Design Consistency Module (DCM) helps diagnose safety concerns at horizontal curves. Crashes on two-lane rural highways are over-represented 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 DCM uses a speed-profile model that estimates 85th percentile, free-flow, passenger vehicle speeds at each point along a roadway. 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 DCM is applicable to both lower-speed highways (i.e., posted speed from 25 to 40 mi/h) and higher-speed highways (i.e., posted speed 45 mi/h and greater).

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.

See IHSDM Library for References.


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