Aimed　at　confusions　on　definition　and　application　of　operating　speed,　authors　discussed　concept　of　operating　speed　and　its　application　in　evaluating　traffic　operation　through　the　field　data　collected　at　various　highways　in　China.　Particularly,　authors　compared　the　characteristics　of　operating　speed　in　China　and　the　United　States　in　terms　of　distribution　of　operating　speed,　drivers＇　compliance　of　speed　limit,　and　investigated　why　the　differential　operating　speed　between　small　and　large　vehicles　in　China　was　much　bigger　than　that　in　the　United　States.　Based　on　the　analysis,　authors　presented　a　close　correlation　between　differential　speed　and　highway　safety.　Results　show　that　the　speed　limit　of　a　highway　must　be　a　function　of　several　factors　including　the　characteristics　of　distribution　of　operating　speed.　In　order　to　enhance　highway　safety,　authors　recommend　the　lane　separation　for　small　and　large　vehicles　under　feasible　conditions.