The design consistency module helps 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.

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