Take a look at all these photos of city streets in the rain. I found them all on google image search and I am shamelessly hotlinking them:

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I was driving home from work today in the rain, and saw many similar scenes. However I noticed something interesting that I have never noticed before. The street lights, car lights, traffic lights, etc. all reflect off of the wet surface of the road. However since the road is not a mirror-flat surface, the reflection is smeared out or diffused due to the rough texture of the road. So my question is this: why is the smearing or diffusion of the reflection almost entirely in the vertical direction? Certainly the surface of the asphalt or concrete does not have a sufficiently heterogeneous asperity (i.e. the roughness is uniform from all directions) to account for this. Shouldn’t the light spread out as far horizontally as it does vertically? Any insights here?