In　this　paper,　we　review　trajectory　data-based　traffic　flow　studies　that　have　been　conducted　over　the　last　15　years.　Our　purpose　is　to　provide　a　roadmap　for　readers　who　have　an　interest　in　the　latest　developments　of　traffic　flow　theory　that　have　been　stimulated　by　the　availability　of　trajectory　data.　We　first　highlight　the　critical　role　of　trajectory　data　(especially　the　next　generation　simulation　(NGSIM)　trajectory　dataset)　in　the　recent　history　of　traffic　flow　studies.　Then,　we　summarize　new　traffic　phenomena/models　at　the　microscopic/mesoscopic/macroscopic　levels　and　provide　a　unified　view　of　these　achievements　perceived　from　different　directions　of　traffic　flow　studies.　Finally,　we　discuss　some　future　research　directions.