なぜ路肩をhard shoulderと言うのか

shoulder自体が柔らかい草でできた路肩のことを指す為。
重いトラックが通る際、ぬかるんで沈んでしまい、かつ道が使いものにならなくなってしまうので、硬い路面、つまりは舗装された”hard”がつくわけですね。
ついでに高速道路で4レーン以上になると1本のみの路肩はあまり避難場所にはなりにくい。というか、ならない。その場でビュンビュン行き交う車をどうにかして回避するしかない。真ん中にやっともう1本整備し始めたのが1968年。


The rise and fall of the second hard shoulderThe hard shoulder, here in the UK, started out as a strip of "reinforced" earth alongside the Preston Bypass , and the first section of the M1 started out with the same. Stories of jacks pushing themselves into the ground instead of lifting up stricken vehicles were rife, and it wasn't long before those shoulders gained a hard top, and the "hard shoulder" became an essential part of the design specification for all new motorways. Far from safety Through the 1960s, roads got wider and wider and plans grew ever more ambitious. As urban planners began pencilling in vast motorway networks, it became clear that many of them would require more than three lanes each way. The first question that arose, in a country that had scarcely any high speed roads with more than even two lanes each way until a decade previously, was: will these roads be safe? The hard shoulder was the place of safety, of course; the thing that made motorways by far the safest type of road. Heads were scratched. If your car made a distressing mechanical sound and black smoke started coming from under the bonnet as you raced along, could you coast across to the far left-hand side of the road quickly and safely? From lanes 1 and 2, yes. From lane 3, yes, just about. From lane 4? Nobody could be sure. A fourth lane seemed too far from the safety blanket. So the design specification, in the late 1960s, was amended. If a motorway had more than three lanes each way, it must have hard shoulders on both sides of both carriageways, four in total. Doubling up As odd as it might sound today, those second hard shoulders (or offside hard shoulders, if you prefer) were a very serious consideration. By 1968, they were a very necessary part of four-lane motorways - not that any had yet been built. In November 1968, under the headline "Fight against London Motorway Box begins", The Times ran a story that quoted Robert Vigars, the chairman of the Greater London Council's planning and transportation committee, describing the urban motorways planned for central London. Even in London where space was at a premium and land was the biggest cost in roadbuilding, the considerable extra width of those extra hard shoulders was considered necessary. "Mr. Vigars said that eight-lane roads with centre hard shoulders would be about 147ft. wide and six-lane roads without them 110ft. wide." The Times , 20 November 1968 Interestingly, the first section of motorway with four lanes on each side is thought to be the M61 through Worsley Braided Interchange , at Linnyshaw Moss near Bolton in Lancashire, which doesn't have second hard shoulders at all. Perhaps, because the four-lane carriageways are relatively short and part of an extended interchange complex, they were thought not to need the extra safety feature.

https://www.roads.org.uk/blog/rise-and-fall-second-hard-shoulder

皆さまが信用極まりないと言うガーディアン紙より
Why is the ''hard shoulder'' so called?

https://www.theguardian.com/notesandqueries/query/0,,-25395,00.html