Why are danger signals in red although the eye is most sensitive to yellow and green?

Although the human eye is sensitive to yellow and green, it has become a tradition to use red as a signal for caution. A ship carrying explosives would have a red flag flying to signal to other ships not to come very close. Experts believe that since the deep red colour of blood could shock people into alertness, such a shade was most suitable as a signal for caution. Red also has the longest wavelength and it was thus believed the colour would promptly draw attention even from a distance.