General Motors Sulfate Dispersion Experiment: Assessment of the EPA HIWAY Model

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Abstract
A major objective of the General Motors Sulfate Dispersion Experiment was to investigate the validity of the Environmental Protection Agency's HIWAY dispersion model. This model was the basis for the high roadside predictions made by EPA in 1975. Comparisons of measurements with predictions show that at the pedestrian level downwind from the road, the model works fairly well under unstable conditions, but overpredicts for stable conditions. The overpredictions get worse as the wind speed decreases, as the wind direction becomes parallel to the road, and as the distance from the road increases. For the upwind dispersion, however, the model Is inapplicable. Under extremely stable or worst meteorological conditions, variable winds at different heights and plume-rise due to the heated exhaust completely invalidate thè model. The overpredictions at the bottom level downwind can be very substantial. When the wind is parallel to the road, overpredictions occur at all sampling positions. New dispersion parameters were determined which substantiate the expectation that mechanical mixing due to the traffic wake completely dominates the effects of atmospheric stability, except under extremely stable conditions. It is believed that the overprediction of the HIWAY model at ground level will become worse as the number of traffic lanes increases.

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