Wind tunnels are fantastic tools that help race teams and manufacturers develop cars that best cut through the air. But testing a stationary car can only do so much. That's where "rolling roads" come in.
A rolling road is exactly what it sounds like. It's like a treadmill, but for cars. A rolling strip of metal sits flush with the floor, allowing the car to rotate its tires and drive in place while being tested in the wind tunnel. By having the wheels spinning, engineers can incorporate an entire extra set of aerodynamic forces affecting downforce and deflection. This makes it easier to see how the car will behave in an actual racing environment.
Ford Performance put together an excellent video detailing how its WRC team used the rolling road wind tunnel to further develop its Fiesta rally cars for the 2018 season. Even if you're not into rally, it's an interesting look into how far wind tunnel tech has come.