Current work zone design and traffic control guidance is based on desirable speed outcomes; knowledge related to actual speed outcomes of design and traffic control decisions is limited. The objective of this research is to investigate relationships among speed behavior, roadway geometrics, and traffic control in freeway construction work zones. Four speed measures are modeled: 85th percentile passenger car speed, 85th percentile truck speed, passenger car speed deviation, and truck speed deviation. The research objective is accomplished through specification and estimation of a first-order autoregressive seemingly unrelated regression model to account for contemporaneous correlations between equation disturbances and positive autocorrelation. Data for model estimation were collected at 119 locations in 17 Pennsylvania and Texas work zones. Model estimation results show that several work design and traffic control features influence speed magnitudes and deviations. Findings also indicate that relationships among speed magnitude, speed deviation, work zone design, and traffic control are more complex than general engineering intuitions expressed in current work zone design guidance.