The capacity and performance of unsignalised intersections are very sensitive to the values of critical gap (headway) and follow-up headway parameters. In particular, in the case of unsignalised intersections controlled by two-way stop and give-way signs, the SIDRA INTERSECTION user needs to calibrate these key parameters to suit local driver characteristics.

A relatively simple method is available to obtain critical gap and follow-up headway estimates from field observations. The method requires queued conditions of the minor (opposed) movement since the critical gap and follow-up headway parameters are relevant to capacity estimation. The method is described in the SIDRA INTERSECTION user guide.

It is recommended that the observed critical gap 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 and follow-up headway as a function of intersection geometry, control and flow conditions.