Speed of advance (or lack thereof due to heavy loads of equipment carried by the infantry) is often cited as a major contributor to the casualty rate. There were many examples, even during the first day of the Somme, where men advanced at the walk and suffered very few casualties. The issue was not how quickly the men could rush across No Man's Land but whether the defenders could be kept suppressed throughout the process. There were very significant differences in the approach to preparatory suppressing artillery fire. In some sectors, the fire was kept on the defenders until the infantry arrived. In many other sectors, the fire lifted off the front line German trenches as soon as the British left their own trenches. No amount of running, even if possible over the distances involved, would have enabled the men to cross against unsuppressed rifle and MG fire.
Aside from keeping artillery fire on the defenders for as long as possible throughout an advance across No Man's Land, there were innovations that helped overcome the issue of getting across the danger zone. In the southern sectors where the British achieved significant success, Russian saps were used to protect the advancing troops. Tunnels were dug across No Man's Land just beneath the surface. These were opened up at the moment of the assault. The other innovation also featured in a successful crossing of No Man's Land. 36th (Ulster) Division troops advanced into No Man's Land during the cover of darkness and lay just outside the German trench line until the barrage lifted.
German MGs were not usually positioned in hardened shelters or emplacements. They were manhandled up from the deep underground shelters and cited in pre-planned positions, often without significant overhead cover. There were a few exceptions, such as the pre-prepared MG positions in the likes of the built-up areas such as Thiepval.
The German artillery response was variable on the day. Bavarian artillery batteries in the sector south of Mametz were all but neutralised by significant counter-battery fire and by the destruction of the deep fire control centre, hit by a French 9" mortar projective. British infantry succeeded with few casualties in this sector.
Due to the threats on Gommecourt and Sere, German artillery had also been moved north from the southern sectors. German artillery fire typically fell on the forming up trenches and communication trenches, often stopping any reinforcements from getting to No Man's Land. Artillery fire on No Man's Land depended on the proximity of the German trench lines, which were quite close in some places and therefore susceptible to blue-on-blue incidents. In other areas, such as near Thiepval, the ground sloped away from the German positions in such a way as to cause significant dead ground that was not easily susceptible to flat trajectory artillery field gun fire.
In the French sectors both north and south of the Somme river, the diversion of some German artillery north was further exploited through the more widespread use of gas shells for counter-battery fire. The British did not have gas shells available for this purpose in July 1916 (French artillery did operate in some British sectors to provide this capability). The French advances were more extensive and made with fewer casualties.
Robert