Question: Which year had the most number of trips that started at stations in San Francisco?
SQL: SELECT SUBSTR(CAST(T1.start_date AS TEXT), INSTR(T1.start_date, ' '), -4) FROM trip AS T1 INNER JOIN station AS T2 ON T2.name = T1.start_station_name WHERE T2.city = 'San Francisco' GROUP BY T1.start_station_name ORDER BY COUNT(T1.id) DESC LIMIT 1