For example, on XV Corps front, the barrage was programmed to lift every minute. Complications arose however in British protocols to prevent friendly-fire casualties which at the time dictated that shellfire was to be kept over one hundred yards away from their own uncovered infantry. In many cases no man's land was narrower than the allowable 'safe' distance and as such the barrage did not protect the men as they went 'over the top' and advanced towards the German trenches. Further, as the British infantry was slowed far beyond the expected pace of advance across no-man's land, all along the Somme front it proved impossible for the infantry to keep up with the pace of the barrage. However, the tactic was further refined as the Battle of the Somme wore on and by September 1916 the creeping barrage became a standard tactic for infantry attacks, and soon spread to the French Army, enabling the French recapture of Fort Vaux at the Battle of Verdun in November 1916. By the later stages of the Battle of the Somme, the British had improved the accuracy of and confidence in their artillery fire and had learned the lessons of keeping infantry close to the barrage: the British Expeditionary Force (BEF) circulated an aerial observer's report commending a "most perfect wall of fire" followed up within by the infantry of 50th (Northumbrian) Division, enabling them to take a village with little opposition. A report said "Experience has shown that it is far better to risk a few casualties from an occasional short round from our own artillery than to suffer the many casualties which occur when the bombardment is not closely followed up". A creeping barrage was essential to the success of the Canadian Expeditionary Force in the capture of Vimy Ridge in April, 1917. The creeping barrage was used during the Action of Tell 'Asur on 12 March 1918 in the Sinai and Palestine Campaign. Six months later, it was used with devastating effect during the Battle of Megiddo (1918) when 18-pdr and Royal Horse Artillery formed a creeping barrage which fired in front of the advancing infantry up to their extreme range while 4.5-inch howitzers fired beyond the barrage, while heavy artillery were employed in counter battery work. The creeping barrage moved at a rate of between , and per minute. At first, British creeping barrages consisted only of shrapnel shells but an equal mix of HE was soon added, in some cases later supplemented by smoke shells. The creeping barrage would advance at a rate of 100 yards every one to six minutes, depending on terrain and conditions; although six minutes was found to be too slow.