For pedestrians, yes that type of road is fairly dangerous. But for cars it's not too bad, drivers know they need to pay attention and nobody is going at high speeds. It's also easier than you might think to see oncoming cars, especially when it's dark and their headlights are on.

Fortunately/unfortunately that type of road is becoming slightly less prevalent, as efficient modern farming means hedges are maintained less than they were historically and they either get chopped down or neglected and end up growing into larger trees that are much easier to see through.

There's usually nothing but fields or woodland behind the hedges. Have a poke about on google maps. https://www.google.co.uk/maps/@51.0386913,-0.0888581,3a,90y,...

While I agree that they're bad for sightlines, they're also traditional and may be on private land, so getting rid of them from minor roads would be a hugely unpopular undertaking.