grammar - How is this ほとんど used?

For full context, see here: https://www.docdroid.net/oraqSdS/img-20170830-0001-new.pdf

The following sentence is taken from Line 2-3: 日本では交通事故で毎年1万人くらい死んでいるが、その原因のほとんどは運転手のミスである。

First, my attempt at translation: In Japan each year around 10000 people are killed in accidents in traffic, but…it is the drivers fault.

I honestly can’t make any sense of その原因のほとんどは .

ほとんど is listed on jisho as an adverbial or temporal noun http://jisho.org/search/%E3%81%BB%E3%81%A8%E3%82%93%E3%81%A9%E3%81%8C I don’t know what a temporal noun is. I know nouns in adverbial function with a temporal meaning. Maybe this is meant here, but I can’t know for sure. And even if I knew what it is, with the english translations provided by jisho I still can’t figure out a meaningful translation.

The same btw. goes for this sentence　(Line 4-5): 飛行機事故、仕事中の事故、家庭での事故、これらの原因もほとんどがヒュマンエラーである。

First, my attempt at translation: „aircraft accidents, accidents at the workplace and accidents at home, ….are human error.“

The part これらの原因もほとんどが is just as cryptic for me. I’d also like to know why も is used here, since the pattern is basically the same as before. Is も used in the function of the の of the sentence before, just with the addition that も emphasizes that the same or a similar thing is mentioned here?

Answer

Xのほとんど means "most of X" or "almost all of X". In this case X is その原因 ("their causes"), so the phrase literally means "most of their causes", or in more natural English "the causes of most of the accidents". I would loosely translate the whole sentence as "Around 10,000 people die in traffic accidents in Japan each year, but the vast majority of these cases are caused by an error on the part of the driver."

The も in the second quoted sentence is a standard "also" usage applying to the 原因, indicating that the causes of all these things too are mostly human error.

Grammatically this も is not replacing a の (which can't generally be replaced by も as it links to another word rather than indicating a grammatical role in the sentence as a whole) but a は. So in this case これらの原因 is the topic of the sentence, but the meaning is roughly the same regardless of whether you say Xはほとんどが… ("As for X, most of them are...") or Xのほとんどが… ("Most of X are...").