Two-lane　highway　is　the　most　general　form　of　highways　in　a　road　network.　Because　horizontal　and　vertical　line　indexes　can　be　chosen　within　a　relatively　large　range,　the　combination　of　them　causes　variable　road　alignments　and　different　capacities.　The　road　alignment　is　one　of　the　main　influencing　factors　on　highway　capacity.　According　to　the　field　data　analysis　and　the　existing　findings,　it　does　not　affect　the　capacity　when　the　radius　is　greater　than　400　m　or　the　slope　is　less　than　3%.　Two　new　indexes　are　defined:　effective　curvature　and　effective　gradient,　and　calculation　methods　are　also　provided.　Effective　curvature　and　effective　gradient　are　divided　into　7　and　6　levels,　respectively,　according　to　the　simulation　results　and　the　principle　of　speed　difference.　Using　the　simulation　method,　the　speed-volume　curve　and　the　volume-following　ratio　relationship　are　obtained　for　42　different　road　alignments.