CREATE TABLE table_46918 ("County" CLOB, "Tunnel" CLOB, "Type" CLOB, "Length  ( Imperial ) " CLOB, "Date of opening" CLOB)

What Tunnel has an Imperial Length of 307 yd?
SELECT "Tunnel" FROM table_46918 WHERE "Length (Imperial)" = '307 yd'
SELECT "Tunnel" FROM "table_46918" WHERE "Length (Imperial)" = '307 yd'