Smiths listed as leaving the country (UK outward passenger lists - shouldn't count military).
1912: 6752
1913: 7093
1914: 4459
1915: 1468
1916: 1118
1917: 383
1918: 345
1919: 2924
1920: 5313
An example: The New York sailed Feb 3 1917 from Liverpool, and reached New York on the 12th. Passengers included women and children, and a 16 year old boy, Clarence Aubrey Smith, heading to work in Montreal. Checking through a few of the names, many seem to be Canadians or Americans heading home - perhaps they figured it was worth the risk of the journey to get themselves safely home and well away from the front.
Several are crossed out, though - those who booked but didn't sail. While in principle the Germans weren't "supposed to" be targeting passenger ships (the Lusitania had been carrying war munitions, and the Germans felt this and other factors qualified treating her as a military target) , I am sure many did not find "supposed to" particularly comforting!
The further drop-of in 1917 is probably largely due to the "Liner Requisition Scheme" as well as the entry of the US into the war.
