The sentence 

#HAS A TYPO

it's just that simple.

> The total length of the public road's network is 29151 km, out of
> which 1243 km are motorways, 6810 km of national roads and 21098 km of
> regional and local.

should be..

> The total length of the public road network is 29151 km, out of
> which 1243 km are motorways, 6810 km are national roads, and 21098 km
> are regional and local roads.

or, if you prefer,

> The total length of the public road network is 29151 km, out of
> which 1243 km is motorway, 6810 km is national road, and 21098 km
> is regional or local road.

It's incredible something so simple can get so much press.

---

Regarding the issue asked about in the comment below.

It couldn't be simpler. You can interpret the three references as to **"sections"** (so for example, the first section discussed is a section 1243kms long), in which case singular.

Or you can interpret the discussion as about the various **"kilometer lengths of highway"** in which case plural.

What's the question?  It's the norm in English that there are at least three ways to go with anything; almost every sentence as given is wholly ambiguous without context.

Note also the infuriating apostrophe-s in "road's" which is just entirely poorly thought out.