A: I am aware that most people are reductionist and like to resolve the one prime thing responsible for the phenomenon bit in reality, multiple things play a part. I shall attempt to be brief in my answer:
The UK driving licence is not trivial and in addition to the theory test, there is a practical component that is at least 40 minutes of driving.
People naturally drive more cautiously and are more aware of their surroundings when the road looks more dangerous. A great example of this would be roundabouts: They look more dangerous than a junction guarded by traffic lights but they're made safe by the drivers being cautious and more aware.
Straight and wide roads invite drivers to pay less attention to the road. They also invite drivers to drive faster. More speed and less attention would naturally result in more road traffic accidents, including single vehicle events where the vehicle enjoys a detour from the road.
Long straight roads are not good for human drivers. They may be great for autonomous vehicles but human boredom should be considered for roads intended for humans. As to why new roads in the UK are rarely straight, it's again because of multiple factors. Sure, an absolutely straight road would be the shortest road but it may not be the cheapest to construct and it may not be safest for human drivers.
