To　measure　and　evaluate　traffic　congestion,　the　baseline　for　calculating　the　cost　of　traffic　congestion　and　methodology　for　measuring　congestion　severity　was　studied.　Combined　with　the　existing　findings,　impacts　of　traffic　congestion　on　traffic　stream　speed,　fuel　consumption　and　emission　of　CO2　were　analyzed　and　the　concept　of　operational　efficiency　was　defined　to　synthetically　describe　traffic　states.　Taking　the　operational　efficiency　as　the　optimization　objective,　the　methodology　of　determining　the　baseline　for　calculating　the　cost　of　traffic　congestion　was　developed.　To　validate　the　feasibility　of　suggested　approach,　the　Van　Aerde＇s　model　was　used　to　describe　speed-density-volume　relationship　and　the　model　parameters　were　estimated　based　on　the　field　data　from　the　4th　ring　road　in　Beijing.　Furthermore,　the　optimal　operational　state　was　obtained　and　the　sensitivity　of　efficiency　to　the　traffic　flow　state　was　analyzed.　Finally,　the　delay　caused　by　congestion,　travel　time　index　and　efficiency　index　were　reformulated　in　the　light　of　congestion　measuring　baseline　suggested　in　this　paper.　©　2022,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.