Drivers have complained for decades about trucks on the road. They go too slow. They take up too much space. They're scary to pass. But the most unsettling aspect of trucks goes beyond what drivers can see: their environmental impact.

Although medium- and heavy-duty trucks account for just 5 percent of traffic, they account for almost one-fourth of all transportation emissions in the U.S. As concerns about climate change and the corporate carbon footprint grow, many companies are trying to limit the environmental impact of trucks.

Tesla, which unveiled its electric long-haul truck last year to much fanfare, was hit with a $2 billion patent suit last month by a firm that claims Tesla stole elements of its hydrogen-powered semi-truck design. Tesla CEO Elon Musk called the suit "laughable" and "absurd" on the company's recent earnings conference call. The real competition, anyway, might come from Big Oil.

Shell Global unveiled its AirFlow Starship in Atlanta in March, and on May 18 the truck — which features a more aerodynamic cab and solar panels on top — will begin a cross-country drive with many stops.