Critical Gap (Headway) and Follow-up Headway are the key parameters in the SIDRA gap acceptance capacity and performance models for roundabouts, intersections controlled by two-way stop and give-way / yield signs and unsignalised pedestrian crossings as well as opposed (permitted / filter turn) movements at signalised intersections.
For model calibration and validation purposes, the gap acceptance survey method needs to match the Critical Gap (Headway) and Follow-up Headway definitions and the assumptions of the gap acceptance model. A SIDRA Gap Acceptance Survey Excel application is available for download. This survey method is recommended for calibration and validation of SIDRA Critical Gap (Headway) and Follow-up Headway estimates.
It is recommended that the observed Critical Gap (Headway) and Follow-Up Headway values are used to calibrate the base values used by the SIDRA INTERSECTION model with the Apply TWSC Calibration parameter checked rather than being used as user-given (constant) values with the Apply TWSC Calibration parameter unchecked. This will allow automatic adjustment of base values of Critical Gap (Headway) and Follow-Up Headway as a function of intersection geometry, control and flow conditions.
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SIDRA Gap Acceptance Survey