Setting　reasonable　speed　limits　is　an　efficient　measure　to　improve　the　safety　of　freeways　in　mountainous　areas.　Twenty-five　typical　sections　randomly　selected　from　Xihan　Freeway　and　Qinglian　Freeway　in　China　were　investigated.　On　the　basis　of　analyzing　the　data　of　25　sections,　speed　distributions　for　cars　and　trucks　under　different　speed　limits　were　obtained.　The　correlation　between　operating　speed　and　posted　speed　limit　of　mountainous　freeways　is　analyzed.　By　means　of　regression　analysis,　the　relationship　model　between　the　85th　percentile　speed　and　posted　speed　limit,　50th　percentile　speed　and　posted　speed　limit,　15th　percentile　speed　and　posted　speed　limit　are　established　respectively.　The　regression　analysis　of　posted　speed　limits　and　operating　speeds　shows　that　there　exists　a　strong　linear　relationship　between　these　two　speed　concepts.　The　15th　percentile　speed　model　is　the　best　fitting　model　among　the　three.　©　2010　ASCE.