Highway photovoltaic roofs; a brilliant idea to decrease carbon emissions and improve road safety

Placement of photovoltaic roofs on the main highways worldwide has the potential to utilise existing land and produce 17,578 terawatt-hours annually, which is over four times the yearly energy output of the United States. According to a study published in Earth's Future, covering the world's highways with solar panel roofs could significantly reduce carbon dioxide emissions and traffic accidents. The ambitious estimate, which calculated the costs and benefits of installing solar roofs on highways around the world, could cut global carbon emissions by approximately 28% by reducing the need for fossil fuels. The study envisions a network of solar panels elevated above highways and other major roads, generating electricity while protecting cars from inclement weather. "There are some highway photovoltaic roof pilot programs, but not on the scale that we have imagined," said Ling Yao, a remote sensing scientist at the Chinese Academy of Sciences and the study's lead author. "Covering Earth's highways with solar roofs could generate 17.58 PWh of electricity per year, accounting for more than 60% of global electricity consumption in 2023. Innovative carbon emission reduction solutions are required to meet the International Panel on Climate Change's 1.5 degrees Celsius (2.7 degrees Fahrenheit) warming goal." 
Solar photovoltaics, or solar panels, accounted for 40% of new renewable energy installations between 2012 and 2021. As the cost of solar panels falls, innovative solar projects like Yao's could aid the global transition to sustainable energy. 

A simple and clever idea 

Yao was inspired to conduct this imaginative study while driving across a flyover on his evening commute. “I suddenly looked down at the road, and thought, ‘The roads are connected to each other. It’s just like a network – the cars can move freely from one place to another,’” he said. “I thought, ‘Why don’t we turn our roads into a network of photovoltaics?’” 
Building solar roofs over highways would use already developed land to generate electricity, reducing demand for greenhouse gas-producing energy and, ultimately, lowering carbon emissions. More than 3.2 million kilometres (1.9 million miles) of highways span the globe, enough asphalt to wrap around the equator 251 times. Solar-roofed parking lots have proliferated in recent years, but photovoltaic-paved and roofed roads are still in their early stages. While some countries have begun small pilot-scale highway photovoltaic projects to demonstrate the technology's potential, long stretches of solar-panelled highways remain a long way off.
According to the study, highway solar roofs have several advantages, including reduced traffic accidents, clean energy generation, and the potential for lower carbon emissions by replacing fossil fuel energy sources. The researchers examined highways and major arterial roads all over the world, calculating the cost of constructing and maintaining a network of solar panels in each country or region. They based their analyses on polysilicon photovoltaic panels with a maximum power generation of 250 watts, tilted 10 degrees towards the highway's outer lanes. 
“Installing solar roofs over the world’s highways and major arterial roads would use 52.3 billion solar panels”, Yao said. The highway-covering solar panels would generate up to 17,578 terawatt-hours per year worldwide, more than four times the United States' annual energy output. Installing solar roofs on major arterial roads would result in an additional 13,570 terawatt-hours per year. The cost per megawatt-hour and potential for energy generation would vary geographically, with the majority of the installations located near densely populated areas with numerous highways, such as eastern China, western Europe, and the United States East Coast.
Installing highway photovoltaic roofs around the world could generate enough energy to replace the equivalent of 9.66 gigatons of fossil fuel-generated carbon dioxide per year, or up to two-thirds of the United States' greenhouse gas emissions in 2022. 
"This truly surprised me. "I had no idea that highways alone could support the deployment of such large photovoltaic installations, generating more than half of the world's electricity demand and significantly easing the pressure to reduce global carbon emissions," Yao said.
Highway solar roofs could also reduce global traffic deaths by 10.8%, particularly in high-precipitation areas, by protecting drivers and the road from rain and snow, according to the study. Improved highway safety would also lower the economic cost of traffic accidents.

What does the future hold?

Yao is hopeful that these small pilot programs will catch on, and that some countries will implement highway solar roofs on a large scale. He envisions photovoltaic roofs being most easily installed on long, flat stretches of highway, like in the southwestern United States.
“Actually, putting this into practice is the best evidence to make someone believe that this idea is practical,” he said. The futuristic highway setup may face setbacks, such as high setup costs (up to four times the cost of equivalent ground-based arrays) and maintenance costs. Beyond those challenges are more mundane issues, such as how to clean billions of solar panels. According to Yao, on the other hand, these small pilot programs will gain traction and that some countries will implement large-scale highway solar roofs. He envisions photovoltaic roofs being most easily installed on long, flat stretches of highway, such as in the Southwest United States. "Actually, putting this into practice is the best evidence to make someone believe that this idea is practical," Yao told.