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As you keep adjusting to the leading car 's erratic speed changes , you sometimes find a clump of vehicles closely tailing you and , at other times , only a few vehicles in sight . That 's great for drawing a map or establishing property lines , but it 's more work that you want to do for just an on-the-fly estimate . The physicists ' findings may be particularly relevant to triathlons - - events in which competitors swim , then cycle and run . These factors generally correspond to everyday situations : mistimed traffic signals , a blocked lane , the narrowing of a freeway to fewer lanes , a busy exit or entry ramp , a truck slowly grinding its way up a hill .