Based　on　definition　of　operating　speed,　experiment　was　performed　and　data　of　real-time　speed　of　the　cars　were　gathered　to　analyze　the　change　of　continuous　speed　of　the　cars　and　the　change　of　the　time　headway　and　the　fluctuation　rules　of　the　speed　differentiae　passing　through　classic　transect　were　studied.　Then,　the　change　and　distribution　characters　of　speed　with　time　headway　were　studied,　and　time　headway　was　regarded　as　the　measurement　index　of　free-flow.　The　judgment　criterion　of　free-flow　speed　was　drawn　using　the　statistical　analysis　method.　Results　show　that　the　time　headway　should　be　more　than　9　s　for　free-flow.　The　conclusions　supply　a　theoretic　base　to　building　of　the　system　of　the　operating　speed　of　highway.