Unless you work in road lighting, would you think about the fact that these lights accumulate dirt over time, decreasing the amount of light that reaches the roadway? And if you would have thought of that, would you have remembered that LED street lights might collect more dirt?
Why would these accumulate more dirt? Because LED lamps last longer than the HID lamps that were used for years to light our roads. So a study looked into this topic and concluded that ... this is a hard thing to study. LED road lights come in many forms, and it's hard to test all brands and all forms. But due to the long lives of LEDs, "luminaire dirt depreciation" (LDD) needs to be factored into lighting plans.
At the very least, planners should think through how the roads will look by end of lamp life with LLD taken into consideration. In a better scenario, operators will plan on cleaning the lights and recapturing the available light, minimizing the impact of LDD. Read more about it on the IES website.