Charlton (2008) conducted a simulated study of drivers’ responses to hazards. The drivers were either driving alone, with a passenger, conducting a hands-free phone call, or conducting a hands-free phone call where the DWLMS emitted a beep when the vehicle approached a hazard. The study found that a driver driving solo was least likely to crash, and a driver conducting a hands-free phone call was most likely. Those who conducted a hands-free conversation using DWLMS performed almost as well as those who were driving solo.