The latest edition of the Highway Capacity Manual (HCM-6) includes, for the first time, a methodology for estimating and predicting the average travel time distribution (TTD) of urban streets. Travel time reliability (TTR) metrics can then be estimated from the TTD. The HCM-6 explicitly considers five key sources of travel time variability. A literature search showed no evidence that the HCM-6 TTR model has ever been calibrated with empirical travel time data. More importantly, previous research showed that the HCM-6 underestimated the empirical TTD variability by 70% on a testbed in Lincoln, Nebraska. In other words, the HCM-6 TTR metrics reflected a more reliable roadway than would be supported by field measurements. This paper proposes a methodology for calibrating the HCM-6 TTR model so that it better estimates the empirical TTD. This calibration approach was used on an arterial roadway in Lincoln, Nebraska, and no statistically significant differences were found between the calibrated HCM-6 TTD and the empirical TTD at the 5% significance level.