To improve work zone safety by replacing human flaggers, a new Automated Flagger Assistance Device (AFAD) developed by the Missouri Department of Transportation (MoDOT) was investigated through field and driving simulator studies. The MoDOT AFAD configuration, including STOP/SLOW paddles, Red/Yellow lights, and a changeable message sign, was incorporated into a truck-mounted attenuator. Driver behavior measures were collected to compare the effectiveness of the MoDOT AFAD, its alternative designs, and human flaggers. In the field study, the MoDOT AFAD was able to slow vehicle approach speeds and stop vehicles farther back than flaggers. In the driving simulator study, the MoDOT AFAD and its alternative designs significantly reduced average approach speeds and increased the distance between completely stopped vehicles and the AFAD/flagger. Surveys conducted after each study showed that drivers understood the AFAD well and preferred it over flaggers, especially the MoDOT AFAD configuration. Overall, this case study demonstrated that the AFAD has the potential to improve the safety of work zones and workers.