Kinda not. Generally if you got road you got reception. Only wilderness areas don’t. For few people who go camping, etc the standalone miniterminal makes most sense.
Yes there are, but I my guess is 95%+ of car miles are in areas with good reception. If you are building roads putting cell reception there is trivial.
Er, if you got highway, maybe? I assure you there are plenty of roads that have poor cell reception.