The　road　tunnel　which　has　advantages　of　improving　the　alignment　criterion,　educing　travel　distance,　protecting　the　natural　environment　is　increased　used　in　China.　At　present,　the　related　domestic　and　foreign　researches　are　mainly　based　on　the　design　scale,　lining　type,　construction　methods　and　difficulty,　use　and　so　on.　There　is　little　about　the　length　definition　of　road　tunnel　based　on　the　drivers＇　driving　safety　and　comfort.　The　driver　is　client　of　tunnel.　The　driving　environment　of　tunnel　is　different　from　the　outside　and　tunnel　length　has　impact　on　the　driving　safety　and　comfort.　So　this　paper　defines　the　length　of　the　tunnel　based　on　driving　safety　and　comfort　in　order　that　the　related　scholars　who　concern　and　study　the　tunnel　treat　them　differently.　By　analyzing　the　safety　and　comfort　test　data　of　62　tunnels,　we　get　the　shortest　tunnel　whose　length　is　450m　based　on　the　evaluation　index　of　driving　safety　and　comfort-workload.　When　the　length　is　less　than　450m,　the　drivers＇　workload　is　low　and　stable,　such　tunnels　have　no　significant　impact　on　the　driving　safety　and　comfort;　When　the　length　is　greater　than　(or　equal　to)　450m,　the　drivers＇　workload　becomes　a　apparent　and　consistent　law　that　it　increases　at　the　entrance　of　the　tunnel,　shows　no　significant　change　in　the　tunnel,　it　increases　again　until　the　tunnel　exit.　The　increased　workload　is　with　a　certain　lead　or　lag.　Because　this　has　a　large　influence　on　the　driving　safety　and　comfort,　we　need　to　consider　some　safe　measures.　©　2010　IEEE.