Hi,
Thank you for your answer, I solved some minutes ago with this code:
usa <- map_data("usa")
ggplot() + geom_polygon(data = usa, aes(x=long, y = lat, group = group),fill="NA",col="black") +
coord_fixed(1.3)+ geom_point(data=dati, aes(x=Start_Lng, y=Start_Lat,col=Severity))+
labs(title = "Fig.6 Map of accidents in America",fill = "Severity") + theme(plot.title = element_text(hjust = 0.5))