Originally it was a speed course. In comparison to other race tracks, at Spa course, the drivers were able to get the highest average speeds. Thanks to those results, the Belgians were filled with proud because of ownership of very fast circuit. Those feelings led to decision to re-design the track in 1939 in order to improve average speeds.

The road near the Eau Rouge brook (known as Ancienne Douane) at the bottom of the valley was interrupted by a faster uphill movement. Today known as the Eau Rouge and Raidillon combination is the most famous part of Spa Circuit. So much popular so that a section of road very similar to the Eau Rouge was created in the Istanbul Park circuit in Turkey. It was jokingly named Faux Rouge (Faux from French means fake).