A computer model, called HIWAY, that can be used for estimating the concentrations of nonreactive pollutants from highway traffic is described. This steady-state Gaussian model can be applied to determine air pollution concentrations at receptor locations downwind of at-grade and cut-section highways located in relatively uncomplicated terrain. The air pollution concentration representative of hourly averaging times at a downwind receptor location is found by a numerical integration along the length of each lane and a summing of the contributions from each lane. With the exception of receptors directly on the highway or within the cut, the model is applicable for any wind direction, highway orientation, and receptor location. The model was developed for situations in which horizontal wind flow occurs. The model cannot consider complex terrain or large obstructions to the flow such as buildings or large trees. An interactive version of the computer model is available on Environmental Protection Agency's Users' Network for Applied Modeling of Air Pollution (UNAMAP).