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

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'