An　urban　network＇s　supply　capability　is　notably　impacted　by　the　operational　efficiency　of　its　bottlenecks,　namely　the　intersections.　An　essential　and　powerful　metric　for　describing　a　network＇s　overall　supply　performance　is　the　capacity　region　(CR),　which　represents　all　potential　combinations　of　capacities　for　different　intersection　movements.　This　study　explores　the　impact　of　imminent　saturation　flow　rate　(I-SFR)　information　on　the　CR　of　traffic　networks.　The　work　is　separated　into　Part　I　and　Part　II　to　prevent　a　lengthy　paper.　Specifically,　in　Part　I,　we　systematically　formulate　the　conflicting　relationship　between　different　movements　using　greatest　conflicting　group　and　green　ratio　constraints,　formally　define　the　dominating　relationship　between　different　CRs,　and　theoretically　demonstrate　1)　how　knowing　the　accurate　I-SFRs　of　additional　conflicting　movements　can　expand　the　CR,　resulting　in　a　larger　CR　that　dominates　the　original　one,　and　2)　how　improving　the　prediction　accuracy　of　observed　movements＇　I-SFRs　can　enlarge　the　CR.　Part　II　will　develop　a　＂ruler＂　-　the　BackPressure　control　with　partial　I-SFR　information　-　to　measure　the　changes　in　CR　and　validate　the　theories　of　Part　I　through　simulation　experiments.