We　study　the　similarities　and　differences　between　the　headway-velocity　relations　under　normal　and　emergency　conditions　to　explore　whether　they　can　be　described　by　a　unified　behavioral　equation.　We　firstly　performed　a　series　of　pedestrian　experiments　in　three　different　scenes　under　normal　and　emergency　conditions　respectively　to　obtain　the　behavioral　parameters　of　headway-velocity　relations　based　on　the　visual　hindrance　field　as　well　as　the　high　precise　trajectories　of　pedestrians.　The　strong　similarities　in　the　headway-velocity　relations　in　both　normal　and　emergency　conditions　suggest　that　a　unified　behavioral　mechanism　is　at　play　in　human-driven　pedestrian　traffic.　This　mechanism　is　essentially　a　safety-driven　self-slowing　behavior　that　pedestrians　try　to　adopt　a　safe　speed　for　a　given　spacing　between　them　to　avoid　collisions　and　preserve　their　personal　space.　We　notice　that　even　in　emergency　escape　situations,　people　still　tend　to　slow　down　the　speed　and　show　defensive　behavior　when　the　headway　approaches　a　critical　value　in　order　to　decrease　the　collision　level　and　protect　the　body　from　contact　as　much　as　possible.　Moreover,　the　differences　between　normal　and　emergency　conditions　are　also　found　in　the　experiments.　Compared　to　normal　condition,　the　free　velocity　of　pedestrians　is　markedly　higher　while　the　minimum　critical　headway　is　much　shorter　in　emergency　situation.　Especially,　the　proportionality　constant,　the　reciprocal　of　which　is　safe　response　time,　is　higher　under　emergency　condition.　That　means　pedestrians　will　slow　down　the　free　speed　to　zero　in　a　shorter　safe　response　time　in　emergencies.　A　modified　social　force　model　is　then　proposed　to　incorporate　this　self　slowing　behavioral　mechanism,　and　different　self-slowing　behavioral　parameters　in　the　model　under　normal　and　emergency　conditions　both　refer　to　the　observed　experimental　data.　Simulations　with　the　same　setup　as　the　experiments　were　carried　out　for　both　normal　and　emergency　conditions　using　the　unified　model　with　self-slowing,　and　the　simulated　spacetime　diagrams　as　well　as　stop-and-go　waves　in　circular　movement　under　normal　condition,　the　simulated　evacuation　efficiency　in　room　evacuation　under　emergency　condition　and　the　simulated　velocity　profiles　in　corridor　scene　under　normal　and　emergency　conditions　all　demonstrate　remarkable　consistency　with　the　experimental　results.　(C)　2017　Elsevier　B.V.　All　rights　reserved.