New research suggests that installing solar panel roofs over highways worldwide could significantly cut carbon dioxide emissions and reduce road accidents. This ambitious proposal estimates that such installations could lower global carbon emissions by around 28% by reducing reliance on fossil fuels. The study, published in Earth's Future, envisions a network of elevated solar panels over highways and major roads, generating electricity and protecting vehicles from adverse weather conditions.

"There are some pilot programs for highway photovoltaic roofs, but not on the scale 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 petawatt hours of electricity annually, which is more than 60% of the world's total electricity consumption in 2023."

Innovative solutions like this are crucial to achieving the 1.5 degrees Celsius warming target set by the International Panel on Climate Change. Solar photovoltaics accounted for 40% of new renewable energy installations between 2012 and 2021. As solar panel costs decrease, creative projects like Yao's could facilitate the global shift to sustainable energy.

Picturing a Renewable and Safe Future

Yao was inspired to conduct this study during his evening commute, when he noticed the interconnectedness of roads. "I thought, 'Why don't we turn our roads into a network of photovoltaics?'" he said. Building solar roofs over highways would utilize already-developed land to generate electricity, reduce greenhouse gas emissions, and ultimately lower carbon emissions. The Earth has over 3.2 million kilometers of highways—enough to circle the equator 251 times.

While solar-roofed parking lots have become more common, photovoltaic-paved and -roofed roads are still in their early stages. Some countries have initiated small pilot projects to demonstrate the technology's potential, but extensive highway coverage with solar panels remains a distant goal.

The researchers analyzed highways and major roads globally, calculating the costs and benefits of building and maintaining a solar panel network in each region. They used polysilicon photovoltaic panels with 250 watts of maximum power generation, tilted at a 10-degree angle toward the outer lanes of the highway.

Installing solar roofs over highways and major roads would require 52.3 billion solar panels, Yao said. These panels could generate up to 17,578 terawatt-hours per year worldwide, more than four times the annual energy output of the United States. Additional installations over major roads could produce another 13,570 terawatt-hours annually.

The cost per megawatt-hour and energy generation potential would vary by region, with most installations near densely populated areas with many highways, such as eastern China, western Europe, and the U.S. East Coast.

By implementing highway photovoltaic roofs globally, the world could replace the equivalent of 9.66 gigatons of fossil fuel-generated carbon dioxide annually, or about two-thirds of the U.S. greenhouse gas emissions in 2022.

"This really surprised me. I didn't realize that highways alone could support 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.

Solar highway roofs could also reduce global traffic deaths by 10.8%, particularly in regions with high precipitation, by shielding drivers and roads from rain and snow. Improved highway safety would also lower the economic costs of traffic accidents.

Challenges Ahead

The futuristic highway setup could face obstacles, including high installation costs—potentially up to four times that of ground-based arrays—and maintenance expenses. Additionally, practical issues like cleaning billions of solar panels must be addressed.

Yao remains optimistic that small pilot programs will expand and that some countries will adopt large-scale highway solar roofs. He envisions these roofs being most easily installed on long, flat stretches of highway, such as those 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.