 Complete the following phrase:  After sitting out the 1973 Isle of Man TT which most of the top racers boycotted due to safety concerns, Saarinen arrived at the Nations Grand Prix leading both the 250cc and 500cc championships. The Monza Circuit, first opened in 1922, was fast and lined with steel barriers which left no room for error for motorcycle racers. The steel guardrails lining the circuit were installed as a result of demands by automobile racers following an accident which occurred during the 