The engineers in the latter part of WW2 were like a well oiled machine, and were renowned for their speed of construction as in Sapper's anecdote.
The film of "Bridge Too Far" shows what happens when the logistics train fails to place the bridging assets where they are needed: right behind the forward troops.
With the equipment in place on site, and reconnaissance and site preparation complete, bridge construction is then very rapid.
In the case of the 2 NZ Division, the engineer regiments were often recruited from the staff of the Public Works Department which had undertaken massive civil projects around New Zealand during the depression years. They built Dams, Power Stations, railway lines, extended the state highway network etc. This was part of a Government initiative to provide relief work for the thousands of unemployed. (Doesn't this sound like what's on the news today?)
After the war the CRE (Commander Royal Engineers) Bull Hanson became the PWD Commissioner of Works, and many sappers returned to the PWD and County Councils putting their skills to use on civil tasks.
The modern civil construction times allow for site selection, survey and preparation, transport of the bridge from a central depot probably hundreds of kilometres away, assembly of a construction team with experienced supervisors from where-ever, building the bridge, then constructing sealed approaches to withstand the size and volume of the modern vehicle fleet.
A week is an incredibly short time for this today!
Rob