for traffic modelling work in the "baseline" scenario, a geh of less than 5.0 is considered a good match between the modelled and observed "hourly" volumes (flows of longer or shorter durations should be converted to hourly equivalents to use these thresholds). according to dmrb, 85% of the volumes in a traffic model should have a geh less than 5.0. gehs in the range of 5.0 to 10.0 may warrant investigation. if the geh is greater than 10.0, there is a high probability that there is a problem with either the travel demand model or the data (this could be something as simple as a data entry error, or as complicated as a serious model calibration problem).