The potential appeal of solar roads - modified solar panels that are installed in place of asphalt - is clear. Generating electricity from highways and streets, rather than in fields and deserts packed with solar panels, could conserve a lot of land. Those advantages are particularly important in a place like China, a heavily populated country where demand for energy has risen rapidly. Now, such roads are finally becoming viable. China's leaders in solar road development are Pavenergy and Qilu Transportation. The surface of these panels, made of a complex polymer that resembles plastic, has slightly more friction than a conventional road surface, according to Zhang Hongchao, an engineering professor. Still, a litany of outstanding challenges means the wide deployment of solar roads is a long way off. Solar roads are ... more expensive than asphalt. It costs about $120 a square meter, or about $11 a square foot, to resurface and repair an asphalt road each decade. By comparison, Pavenergy and Colas hope to be able to bring the cost of a solar road to $310 to $460 a square meter with mass production. Panels on a highway would likely need to be replaced less often than asphalt, Professor Zhang said. And a solar road can produce about $15 a year worth of electricity from each square meter of solar panels. Less clear is whether the panels would be able to take the pounding of millions of tires each year for more than a decade, or whether they might be stolen.