Safe for racing

All Formula 1® circuits are surfaced with asphalt. Racing cars and motorcycles require a durable, predictable and smooth surface. Asphalt is the only surface that is suitable for the rigorous requirements of the pinnacle of motorsport. The circuit designers must take into account not only the extreme performance of the cars, but also the safety of drivers, riders and spectators. The excellent friction properties of asphalt also provide grip for the extreme cornering angles experienced during motorcycle racing, a lesser surface than asphalt would cause them to slip as if they were riding on soap.

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Ref.
Formula 1 circuits – layer by layer; Beneath the surface of a Formula 1 track; FIA official website http://www.formula1.com/news/features/2006/11/5333.html