Objective:　This　article　examines　the　effects　of　fatigue　on　driving　performance　under　different　roadway　geometries　using　a　driving　simulator.　Methods:　Twenty-four　participants　each　completed　a　driving　scenario　twice:　while　alert　and　while　experiencing　fatigue.　The　driving　scenario　was　composed　of　straight　road　segments　and　curves;　there　were　6　curves　with　3　radius　values　(i.e.,　200,　500,　and　800m)　and　2　turning　directions　(i.e.,　left　and　right).　Analysis　was　conducted　on　driving　performance　measures　such　as　longitudinal　speed,　steering　wheel　movements,　and　lateral　position.　Results:　Results　confirmed　that　decremental　changes　in　driving　performance　due　to　fatigue　varied　among　road　conditions.　On　straight　segments,　drivers＇　abilities　to　steer　and　maintain　lane　position　were　impaired,　whereas　on　curves　we　found　decremental　changes　in　the　quality　of　longitudinal　speed　as　well　as　steering　control　and　keeping　the　vehicle　in　the　lane.　Moreover,　the　effects　of　fatigue　on　driving　performance　were　relative　to　the　radius　and　direction　of　the　curve.　Fatigue　impaired　drivers＇　abilities　to　control　the　steering　wheel,　and　the　impairment　proved　more　obvious　on　curves.　The　degree　varied　significantly　as　the　curve　radius　changed.　Drivers　tended　to　drive　closer　to　the　right　side　due　to　fatigue,　and　the　impairment　in　maintaining　lane　position　became　more　obvious　as　the　right-turn　curve　radius　decreased.　Conclusions:　Driver　fatigue　has　detrimental　effects　on　driving　performance,　and　the　effects　differ　under　different　roadway　geometries.