The skulls or "curega" as they call on the tournament, are often used to describe the level of danger that part of the race has.

Zero Skull = Enjoy the ride!
One Skull = Watch-out, be careful!
Two Skulls = Be really careful at this point!
Three Skulls = Seriously, be extremely careful at this point, if you don't, it might end-up badly!

When I started the job, one of the request was to bring elements and colours that the people in the area would be able to associate with rally racing at once.