It is most difficult to judge the speed of other vehicles when they approach from the front in the dark.
Humans can judge distances to a certain extent but are considerably worse at judging speeds. We are poor at determining the speed of oncoming vehicles, but much better at judging the speed of vehicles approaching from the side, for example a car that is driving on a crossing road.
To judge and calculate distances and speeds, the brain uses a variety of clues and details that it interprets and analyses. For example, telephone poles along a road can help the brain estimate the distance to a particular point further away, and how quickly an oncoming car grows in size gives an indication of its speed.
If these clues and details become unclear due to darkness, rain, snow, fog or the like it will be much more difficult for the brain to make accurate assessments.