In　recent　years　with　the　development　of　highway　construction,　traffic　safety　problem　becomes　more　serious　in　China.　A　main　cause　of　accidents　is　the　unreasonable　design　of　roadway　which　induces　drivers　to　make　errors.　The　value　of　the　side　friction　factor　is　a　base　design　parameter　for　highway　curves.　This　paper　explored　quantitatively　rules　of　passenger　and　driver　comfortableness　under　different　speeds　and　different　curve　radii　with　physiological　psychology　experiment.　Then　authors　made　a　choice　of　a　threshold　value　of　the　side　friction　factor　which　accorded　with　characteristics　of　physiological　and　psychological　of　drivers　and　passengers.　Finally,　the　paper　had　proposed　the　important　design　parameter　values　of　curve　at　different　speeds　corresponding　the　threshold　value　of　the　side　friction　factor,　such　as　the　minimum　limit　radius　of　curve.　As　a　result,　the　research　would　contribute　to　guaranteeing　design　consistency　and　making　alignment　design　more　comfortable　and　safe　to　reduce　traffic　crash.