... Selective-yellow light can improve a driver's ability to see in fog or rain or snow, but not because it 'penetrates fog better' or 'reflects less off droplets' as is commonly thought. That effect is known as Rayleigh Scattering, and is why the sky appears blue. However, it occurs only when the droplet size is equal or smaller than the wavelength of the light, which is certainly not the case with ordinary fog, rain or snow. Roadway Fog droplets are several orders of magnitude larger than visible light wavelengths—no Rayleigh Scattering.

So, why do yellow fog lamps work better? It's because of the way the human eye interacts with different colors of light. Blue and violet are very difficult for the human optical system to process correctly. They are the shortest visible wavelengths and tend to focus in front of our eyes' retinae, rather than upon it. To demonstrate this to yourself, find a dark blue store front sign or something else that's a dark, pure blue against a dark background in the absence of white light. From any appreciable distance, it's almost impossible for your eyes to see the blue lighted object as a sharply defined form...the edges blur significantly.) Blue also is a very difficult color of light to look at if it is at all intense...it stimulates the reaction we call "glare". So, culling the blue out of the spectrum lightens the optical workload and reduces glare. For more in-depth discussion of the matter, see Bullough & Rea's excellent study on the topic.