Pavements typically make up 30% to 45% of the land area in major cities and are known to contribute to the urban heat island (UHI) effect since they have high levels of thermal storage and a low reflection of solar radiation.
Conventional paving materials can reach peak summertime temperatures of 120°–150° F, transferring excess heat to the air above causing the areas to feel much hotter than normal.
Due to the large area covered by pavements in urban areas, they are an important element to consider in reducing the heat island effect.
Cool pavements can be created with existing paving technologies (such as asphalt and concrete) as well as with newer approaches such as the use of coatings or grass paving. To help address the growing demand for guidance on pavement choices, the Transportation Research Board has formed a subcommittee on Paving Materials and the Urban Climate. The subcommittee's scope includes modeling, design practices, testing, standards development, and planning and policy considerations.
Finding pavements that are as affordable and as durable as asphalt, however, has proven difficult.
Don’t Paint it Black
Scientists say dark colors like asphalt have been known to absorb over 90% of the sun’s radiation, storing it there for hours, even overnight.
Los Angeles is currently testing a new pavement called CoolSeal, a light gray material that is designed to reduce pavement temperatures, which skyrocket during Southern California’s brutal summer heat waves.
Los Angeles Street Services is working with asphalt coating maker GuardTop LLC to test the cool pavement, which was installed two years ago at the Balboa Sports Complex parking lot.
Due to the pavement having a lighter color surface than traditional street coatings, the lot’s average summer temperature dropped by about 20° F after CoolSeal was applied, according to the company.
The Jordan Avenue project, which is in one of Los Angeles’ warmest neighborhoods, is the first application of the pavement on a public road in California.
It’s just one part of 14 pavement tests to be completed by the end of June in other council districts.
City officials said the coating could cost about $40,000 per mile and last up to seven years before reapplication is needed.
Other lighter alternatives like concrete can lower ambient temperatures in cities. The materials in concrete and the manufacturing involved, however, typically make it more expensive than asphalt — and paving an asphalt road is not cheap in the first place.
In 2014, the Florida Department of Transportation estimated that repaving just one mile of a four-lane urban roadway costs the state $2,413,168. The same year, the Arkansas State Highway and Transportation Department estimated that paving a mile of one-lane road in asphalt costs $700,000, while the same stretch paved in concrete would cost $1,000,000.
Rather than abandoning asphalt — which is an unlikely proposition in many places — researchers think cities can integrate paler crushed rocks into the asphalt pavement. With black tar as a binding agent, this won’t achieve the reflective nature of the CoolSeal.
Even if lighter pavements did become durable and affordable, some experts say it wouldn’t necessarily make a big difference in city temperatures. The amount of greenery and natural spaces in cities are also influential factors as they absorb less solar heat and shade pavements from the sun which has a cooling effect.
This is why cities are also looking in to more grass paving, which is essentially a structure which provides load bearing strength while protecting vegetation root systems from deadly compaction, while decreasing heat island effect. High void spaces within the entire cross-section also enable excellent root development and storage capacity for rainfall from storm events.
However, due to the cost of these alternatives, city planners may end up sticking with traditional asphalt for many years to come, even if it is hot to the touch. When some cities can’t afford to keep their streets paved as it is, it’s difficult to argue with anything as affordable and reliable as asphalt.