<P> Because of concerns about unauthorized access to underground spaces, manhole covers may be locked down, or even temporarily spot - welded in place . This practice has become routine in some locales, as advance preparation for official parades and similar events attracting large crowds or important people . </P> <P> In urban areas, stray voltage issues have become a significant concern for utilities . In 2004, Jodie S. Lane was electrocuted after stepping on a metal manhole cover, while walking her dog in New York City . As result of this and other incidents, increased attention has been focused on these hazards, including technical conferences on stray voltage detection and prevention . </P> <P> Because of their aerodynamic design, some modern racing cars create enough vacuum to lift a manhole cover off its recess . During races on city streets, manhole covers must therefore be welded or locked down to prevent injury . In 1990, during the Group C World Sportscar Championship race at Circuit Gilles Villeneuve (located in a public park in Montréal, Quebec), a Brun Motorsport Porsche 962 struck a manhole cover that was lifted by the ground effect of the car he was following, a Courage C24 Porsche . This caused the trailing Porsche to catch fire, and safety issues ended the race shortly afterwards . </P> <P> According to urban legend, a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at high enough speed to achieve escape velocity . The myth is based on a real incident during the Operation Plumbbob nuclear tests, when a 900 - kilogram (1,984 lb) steel plate cap was blasted off the test shaft at an unknown speed, and appears as a blur on a single frame of film of the test; it was never recovered, but it likely burned up in the atmosphere due to friction . A calculation before the event gave a predicted speed of six times Earth escape velocity, but the calculation is not likely to have been accurate . After the event, Dr. Robert R. Brownlee described the best estimate of the cover's speed from the photographic evidence as "going like a bat out of hell!" </P>

Why roads round about are of circular shape not of any other shape