The　relationship　between　posted　speed　limits　and　operating　speed　is　one　of　the　main　factors　in　determining　appropriate　speed　limits　in　a　speed　zone.　In　this　research,　19　sites　were　randomly　selected　from　the　3rd,　4　th,　and　5th　Express　Ring-Road　in　Beijing,　and　the　spot　speed　and　headway　data　were　collected　at　these　sites.　Then,　free-speed　data　were　reduced　from　the　original　data,　and　the　85th　percentile　speeds　were　computed　as　each　site＇s　operating　speed.　Deviation　analysis　of　operating　speed　at　the　3rd　and　4th　site　shows　that　there　were　no　obvious　distinctions　between　these　two　types　of　road　facilities.　The　regression　analysis　of　posted　speed　limits　and　operating　speed　shows　that　there　exists　a　strong　linear　relationship　between　these　two　speed　terms.　The　percentage　of　operating　speed　exceeding　posted　speed　limits　increases　with　the　speed　limit　value.　The　relationships　between　speed　limits　and　operating　speed　established　here　may　be　expected　to　provide　some　quantifying　basis　for　transportation　agencies　to　set　posted　speed　limits　in　speed　zones.　Results　of　this　study　may　help　the　Department　of　Transportation　and　its　districts　quantify　the　speed　limits　and　provide　more　objective　insights　for　setting　speed　limits.　©　2009　ASCE.