In　response　to　developing　and/or　diminishing　foggy　conditions,　the　variable　speed　limit　application　in　a　connected　vehicle　environment　(CV-VSL)　can　estimate　and　deliver　recommended　travel　speeds　to　individual　drivers,　which　can　help　to　reduce　crashes　when　visibility　conditions　change.　This　study　aims　to　quantify　the　effectiveness　of　the　CV-VSL　application　by　exploring　drivers＇　reactions　to　warnings　(e.g.,　recommended　travel　speeds).　In　order　to　analyze　the　effectiveness　of　the　CV-VSL　application,　a　connected　vehicle　testing　platform　was　established　based　on　a　driving　simulator,　and　characteristics　of　the　drivers＇　speed　adjustments　after　receiving　warnings　were　analyzed　with　respect　to　different　levels　of　visibility　(i.e.,　no　fog,　slight　fog,　and　heavy　fog).　This　study　also　examined　the　effect　of　warnings　on　drivers　in　different　impact　zones　(i.e.,　clear　zone,　transition　zone,　and　fog　zone).　Three　indicators　were　identified:　1)　speed　at　the　end　of　the　clear　zone,　2)　maximum　deceleration　rate　in　the　transition　zone,　and　3)　average　speed　reduction　in　the　fog　zone.　Throughout　the　experiment,　the　relationship　between　speed　adjustments　and　the　level　of　visibility　was　explored.　The　results　indicated　that　the　CV-VSL　application　is　effective　in　making　drivers　reduce　travel　speeds　in　all　three　types　of　zones.　Furthermore,　it　appeared　that　the　CV-VSL　application　could　help　manage　travel　speeds　prior　to　vehicles　entering　the　transition　zone,　and　influence　drivers＇　braking　decisions　upon　encountering　reduced　visibility.　It　was　also　found　that　the　CV-VSL　application　was　more　effective　in　heavy　fog　conditions　than　in　light　fog　conditions.　The　connected　vehicle　testing　platform　based　on　the　driving　simulator　provided　a　new　method　for　evaluating　the　effectiveness　of　in-vehicle　messaging　generated　by　connected　vehicle　applications.