By the early 1930s, the increasing speeds began to make the track more dangerous. In the period 1931–1935 there were 15 fatalities. Part of the bricks were replaced with tarmac (a tar covered macadam or small stones). During the 1935–1936 seasons a number of changes were made. The inside wall was removed in the corners. The outside wall angle was changed to help keep cars inside the track. Hard crash helmets became required. The first yellow lights were installed around the track.