Automated flagger assistance devices (AFADs) are designed to improve worker safety by replacing flaggers who are typically located near traffic approaching a work zone. In this study, a new AFAD developed by the Missouri Department of Transportation (MoDOT) was evaluated via a combined driving simulator and field study. The MoDOT AFAD configuration conformed to the Manual on Uniform Traffic Control Devices and involved STOP/SLOW paddles, red/yellow lights, and a changeable message sign (CMS). This AFAD was incorporated onto a truck-mounted attenuator for operator protection. Driver behavior measures, including approach speed, initial braking location, full stop distance, reaction time, and intervention rate, were used to measure the effectiveness of AFAD as compared to a human flagger. In the field study, the AFAD induced slower vehicle approach speeds (4.20 mph less), stopped vehicles farther back (11.4 feet), and released traffic quicker (1.3 seconds less) than flaggers. In the driving simulator study, the AFAD and its alternative designs significantly reduced average approach speeds (7.7 to 8.9 mph) and increased the distance at which the approaching vehicles came to a complete stop (24 to 48 feet). Both the field and the simulator study were followed by surveys that captured driver preferences and understanding. The results from both surveys showed that drivers understood AFADs well and preferred AFADs over human flaggers, especially for the MoDOT AFAD configuration. Overall, the AFAD has potential to improve the safety of work zones for both workers and the traveling public.