Safety requirements include the use of suspended wire fences, air fences and wooden fences. Air fences are made up of inflated panels installed on the bends. The fence is designed to dissipate energy by allowing an impacted area to compress and transfer air into the rest of the fence through blow-off valves or restriction ports connected to the other sections. They are mandatory for tracks in the British Elite League, Polish Ekstraliga, Speedway Grand Prix and Speedway World Cup. Neutral zones outside the track provide safe run-off areas for riders and their machinery to minimise the potential risk of injury to spectators. The majority of tracks are dedicated to speedway or other sports such as sidecar speedway and banger racing. For larger events, the FIM occasionally sanctions the use of stadiums that install temporary tracks such as the British Grand Prix held at the Millennium Stadium in Cardiff.