Question: how many trips started from south van ness and ended at san fransisco
SQL: SELECT count ( * )  FROM station AS T1 JOIN trip AS T2 JOIN station AS T3 JOIN trip AS T4 ON T1.id   =   T2.start_station_id AND T2.id   =   T4.id AND T3.id   =   T4.end_station_id WHERE T1.city   =   "South Van Ness" AND T3.city   =   "San Fransisco"