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Speech communication from the front seat to the rear seat in a passenger vehicle can be difficult. This is particularly true in a vehicle with an acoustically absorptive interior. Speech Transmission Index (STI) measurements can quantify the speech intelligibility, but they require specialized signal processing. The STI calculations can be simplified if it is assumed that reverberation and echoes play an insignificant role in an automobile. A simplification of a STI measurement is described that uses a stationary reference speech signal from a talker mannequin in the driver's seat to create a signal at the rear passenger positions. On-road noise measurements are used for the noise level and the calculated signal to noise ratio is used to calculate a simplified STI value that tracks closely to a full implementation of the STI method for sedans. In fact, this method is very similar to the techniques described in the Articulation Index (AI) and Speech Interference Index (SII) standards.