It is always hard at night.
There is no doubt in my mind that it is harder to judge vehicle speeds at night time.
I am actually considered an "expert" witness in courts of law in Western Australia by virtue of my training and experience in this field and courts generally accept the evidence of speed estimation by expert witnesses.
Another point to watch out for is "bend syndrome" where vehicles always appear to be travelling faster than they really are when travelling towards you around a bend.
As a part of giving evidence on radar/Laser readings obtained, police officers also always give evidence of their estimation of the vehicle's speed.
This has a two-fold effect.
Firstly, it helps to validate the radar or Laser reading's accuracy and secondly, it provides an opportunity for the court to convict even if the technical aspect of the prosecution evidence in relation to the radar or Laser fails.
Police officers are trained in such a way so that if their own estimation of a vehicle's speed is not consistent with the reading obtained on an authorised speed detection device, then don't prosecute on that speed detection device's displayed reading.