Brightly painted asphalt was supposed to be a breakthrough in the fight against urban heat islands. Los Angeles has implemented such a solution on hundreds of kilometers of streets, investing millions of dollars. However. doubts are arising.
You've probably heard of many cities painting the surfaces of their streets white. The most famous is Los Angeles, which launched the action in 2015, and in the following years other localities joined similar measures. LA plans to cover more than 400 km of roadway strip by 2028.
Mayor Eric Garcetti's initiative aimed to reduce the city's temperature by 1.67°C over the next 20 years. Part of the plan was to cover many black roads in Los Angeles with a reflective white coating, originally developed by the military to keep spy planes cool and hidden from infrared satellite cameras. Traditional dark asphalt absorbs 80 to 95 percent of the sun's rays. That's why, in hot summers, we can sometimes leave a shoe in the molten smear while crossing the street - this one heats up to 65°C. The coated streets reflect most of the sun's radiation and are on average approx. 5-8°C cooler than standard asphalt. The overlay promised to reduce temperatures in the area by up to 5.55°C! Los Angeles therefore invested nearly $25,000 for each kilometer of street, covering it with such a coating. The shelf life of such a solution was supposed to cover about seven years.
LA are painting roads white to reduce city temperatures: https://t.co/hkQENzpyP4 #innovation #construction #cities pic.twitter.com/sNF4U4lcQ3
- Fred Mills (@Fred_Mills) June 6, 2018
verification of assumptions
10 years later, we can now say that, however, according to the latest research, these types of pavements may be cooler on their own, but they can actually make people feel much more warm.
Why? Scientists found that reflected sunlight did not disappear, but... was absorbed by people. This means that on a hot, dry day, a person can feel more than 7 degrees more heat in such a place compared to ordinary asphalt pavement. Unfortunately, this tool, intended to help solve the problem of urban heat islands, seems to have an undesirable side effect. On a typical summer day, with a maximum air temperature of 31°C, the average radiant temperature on such streets was more than 7°C higher before noon compared to black pavement, and in the afternoon the difference remained above 3°C. This was mainly due to solar radiation reflection, which in the case studied was 130 W/m², corresponding to an additional about 10% of direct sunlight. Measurements carried out on exposed, shadeless streets confirm that the effect of reflected radiation can worsen the thermal comfort of pedestrians, increase sweating and the risk of dehydration, and in the case of vulnerable groups - promote serious health problems, including heat stroke.
Given the high number of dry and sunny days in Los Angeles and the limited amount of shade, the findings raise concerns about the validity of coatings in areas with high pedestrian traffic. They may yield better results in locations with few pedestrians, such as extensive parking lots or industrial areas. In doing so, the researchers point out that the problem of overheating in residential parts of the city can be reduced by planting trees along such streets.
Does it clothe and harm?
It is also worth mentioning that such materials also reflect ultraviolet (UV) radiation, which is a major cause of melanoma and other skin cancers. In addition, it causes burns, accelerated skin aging and eye damage. Creating completely bright spaces can increase UV levels by up to 300 percent compared to non-reflective surfaces.
Research conducted by experts at RMIT University and the SunSmart program has shown that the issue of UV exposure is rarely addressed in urban planning policies. Dark surfaces such as asphalt absorb most of the sun's radiation, making them heat up strongly, while materials such as glass, metal, but also light coatings - reflect both heat and UV rays.
Australia has measured this
The problem is particularly relevant in Australia, which has the highest melanoma rates in the world - more than two-thirds of the population will receive a skin cancer diagnosis in their lifetime. More than 15,600 new cases of melanoma were reported there in 2019, and the number continues to rise. An analysis of the health documents of all 79 local government units in the state of Victoria showed that sun protection provisions focus mainly on reducing urban heat rather than UV risks. Seven local and state planning policies were also analyzed, which identified 102 provisions for creating shade or changing pavement materials. Of these, 21 referred to surfaces - primarily changing the color of pavement to reduce temperatures. None of the documents, however, contained references to UV radiation or skin cancer.
Let's return to Los Angeles, however.
Since the beginning of the program, hundreds of kilometers of so-called cool pavements have been built in Los Angeles. Some residents are noticing improvements. The effectiveness of such pavements is being studied by scientists, including Kelly Turner, a professor of urban planning and geography at the University of California. She points out that "cool streets" do a good job of reducing the amount of heat absorbed by asphalt, especially in the afternoon, when there is no "hot stove" effect. However, at noon, between 11 a.m. and 1 p.m., the surface reflects heat toward objects above it, confirming the claim that pedestrians will then feel even worse. The effectiveness of the technology thus depends on the time of day. Turner notes that it won't work, for example, around schools, where children play at noon, but in residential areas - when residents are returning from work and walking their dogs - it can have a noticeable effect.
When in our country?
Although the trend of painting streets with bright coatings has not yet emerged in Poland, it is worth knowing the results of research before we decide to introduce such a solution on a mass scale. Before grabbing rollers and spending millions, it's worth listening to what the research says: cooler asphalt can in practice mean a hotter feel for pedestrians and a higher dose of harmful UV. If we're already experimenting, it's more likely to be on rooftops, where reflective coatings don't glaring or reflecting rays straight at people. And let's remember that the best "natural air conditioner" grows before our eyes - it's greenery, only that these still too often lose out to concrete and the fashion for "modern" spaces. And when she disappears, it may turn out that painting the streets was just another expensive substitute for something we had for free.
