They could get to know each other without supervision.
and
They had a cheap fast mode of transportation to go to places and to get to school, home and work without the expense of a car.
The answer is Literacy increased among ordinary people.
I searched that just to make sure that I was right.
Well the use of trenches prolonged WWI because it was another kind of land warfare where most of the ground in which they were fighting consisted mostly trenches. These trenches protected them from most artillery and normal gun fire. So it was almost like a stalemate.