When we think of a road, our minds conjure up images of obsidian black asphalt. However, in sunny states such as California, serious consideration is being given to changing it to a white or light gray tone. This is not an aesthetic choice, but rather a radical change in the fight against one of the most dangerous enemies of modern metropolises: extreme heat.
Los Angeles may seem like a dream city… until summer arrives and we have to deal with historically high temperatures. Being stranded with a dead battery in the middle of the road can pose a risk of dehydration and heatstroke. That’s why Los Angeles has taken on the role of a pioneer city and decided to become the first US city to experiment with what is known as “cool pavement.” Traditional black asphalt is being replaced by a reflective coating that promises to reduce its surface temperature by up to 15°F.
The heat island effect
They have been seriously considering it for years, but it was not a simple solution… and certainly not a cheap one. Replacing traditional asphalt costs no less than $40,000 per lane mile. However, there is much to be gained by improving the impact on urban life.
The “urban heat island effect” is something that plagues all cities with too much sun and too little vegetation. The building materials used in cities—concrete, brick, and asphalt—absorb and retain much more solar heat than the surrounding natural landscapes, such as parks or forests. You’ve probably noticed it before: it’s scorching hot all day, and when you go out for a walk at night, the heat seems to linger around the ground floor of buildings, as if it were an oven.
This is the island effect; when the heat that has been concentrated on all the surfaces created by humans has dissipated, the sun comes out again and the cycle begins anew.
One of the main culprits of this thermal phenomenon is black asphalt. It is a material with very low reflectivity; it absorbs a significant amount of solar energy, between 80 and 95% of the sun’s rays. This massive absorption of energy raises the surface temperature of streets to dangerous levels on hot, sunny days, reaching up to 150°F (approximately 65°C). That is why dog owners are often advised to check the temperature of the ground where they are walking: if you cannot keep your hand on the asphalt for 10 seconds without getting burned, your dog will suffer and its paws will get burned.
If you multiply this by streets and streets, the consequences are serious: the heat radiates into the atmosphere, raising the ambient temperature throughout the city, often by between 2 and 10° more than in rural areas. This makes it necessary to use air conditioning, which in turn uses more energy, pollutes, and expels even more heat outside buildings. Needless to say, this extreme heat also damages infrastructure and causes accelerated degradation of roads and asphalt itself.
The solution: painting the asphalt
To combat these unbearable cycles of extreme heat, Los Angeles has launched Cool°Streets LA, which aims to provide a high-tech solution with low visual impact. To do this, they will use a special water-based reflective coating called CoolSeal—initially developed for military purposes—which will be applied to existing pavement.
Being a lighter material, cool pavement reverses the dynamics of black asphalt, and instead of absorbing so much solar energy, it reflects it back into the atmosphere. It is estimated to have a useful life of between five and seven years before requiring reapplication. Although the long-term goal of cities is to reduce their average temperature to mitigate health risks to the population and reduce the demand for air conditioning, we still do not know when it will begin to be used.