The　purpose　of　this　paper　is　to　study　the　calculation　methods　of　intersection　unblocked　reliability　based　on　the　random　arrival　of　vehicle　Using　theories　of　probability　and　traffic　flow,　with　primary　consideration　of　random　arrival　of　vehicle,　the　unblocked　threshold　of　the　urban　signal　intersection　is　determined　by　analyzing　the　intersection　vehicle　arrival　distribution　model.　Besides,　the　model　is　verified　with　simulation　software　Paramics.　This　study　proposes　a　new　approach　for　calculation　method　of　unblocked　reliability.　(C)　2013　The　Authors.　Published　by　Elsevier　Ltd.