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ABSTRACT (150-250 words) Efficient and reliable public transit systems provide opportunities to reduce congestion, emissions in urban areas and provide access and mobility to residents. Headway, or the time difference between departing or arriving vehicles, is a useful measure to gauge bus transit performance; because short headways can lead to bus bunching incidents that quickly degrade transit level of service. While Transit Signal Priority (TSP) has been shown to decrease travel time and delay experienced by buses, little work has shown how TSP may affect bus bunching. This research attempts to understand the characteristics of bus trips, especially TSP, that prevent or promote short headways, or bus bunching. A study of two transit routes in SE Portland is presented. High values of negative serial correlation were observed among consecutive headway observations. A regression model is used to analyze factors (boardings, alightings, stops, lift usage, Transit Signal Priority and direction of travel) that may influence bus headways. Priority requests are shown to have a significant effect on headways