The primary reason why the color red is used for danger signals is that red light is scattered the least by air molecules. The effect of scattering is inversely related to the fourth power of the wavelength of a color. Therefore blue which has the least wavelength of all the visible radiations is scattered the most and red which has the highest wavelength of all the colors we can see is scattered the least. So red light is able to travel the longest distance through fog, rain, and the alike.
Also, red is a color we inherently perceive as one that is associated with danger. I guess it has been genetically programmed as hot objects are red in color and also red is the color of blood. These reasons make danger signals the most effective when they are painted in red.
Among the three basic colours - red, yellow and blue - as well as other common colours such as green brown and orange the colour red is the one that was found to be most easily noticed by humans. This made it traditionally the most suitable colour for drawing attention of people. Therefore, the colour red has been used in preference over other colours, for many centuries in situations, including danger situations, where attention of people was to be drawn urgently and effectively. Over a period this practice of using colour red for drawing attention to thing and situations posing danger has made red a socially accepted symbol of danger around the world, further increasing is effectiveness as a symbol of danger.