When the latest section of the Waikato Expressway (Horotiu to Hamilton airport turn off) opened I noticed several sections where there was a noticeable difference in the road noise. There was a visible difference as well even though the smoothness/finish looked to be the same. The visible difference was more akin to the mowing lines you see on a football field.

The difference in road noise piqued my curiosity and I wondered what was causing the difference when there was no noticeable difference in the surface texture/roughness. Obviously there is a difference between both surfaces but it is hard to see.

Today I noticed another difference. The amount of surface water on the sections that are noisier to drive on. This water is sprayed about by preceding vehicles, especially trucks, severely reducing visibility. It's hard to see past them as you overtake. Yet on the other quieter sections there is a lot less spray/wash from preceding vehicles and visibility is vastly better. The noisy sections have a glossy look to them while the quieter sections have a matt black look.

Can anyone explain why there is this difference? 

It would be great if the quieter sections with less spray were used everywhere.