AbstractConnected and automated vehicles (CAVs) will potentially improve transportation system performance. Guidance on the capacity impact of CAVs at different market penetration rates (MPRs) will help agencies to incorporate the effects of CAVs when planning and designing roadways. Traditionally, practitioners have used the Highway Capacity Manual (HCM) to assess capacity and evaluate the quality of service for various facilities and systems. Although several studies have provided insight into the capacity benefits of CAVs, there is a need for quantified CAV effects that can be used to develop HCM guidance. In this study, the capacity benefits of CAVs at signalized intersections were estimated, and capacity adjustment factors (CAFs) were developed for the HCM. The researchers considered variations in CAV gap/headway settings, platoon lengths, turning movement types (through and left), and left-turn phasing modes (protected versus permitted). Microscopic traffic simulation was used to model CAVs. The results showed that performance indicators such as saturation headway gradually improved with increases in CAV MPR, resulting in up to 40% capacity increase at 100% MPR for the protected movements. For permitted left turns, up to 45% capacity increase could be achieved at 100% MPR. This increase in permitted left-turn capacity can be attributed to vehicle-to-vehicle (V2V) communication, which provides advanced information on available gaps in conflicting traffic and reduced follow-up headway time both for permitted left turns and the opposing through movement. Based on the capacity results, this study provides CAF tables for CAVs that can be easily integrated into the HCM and used for planning-level guidance by practitioners.