Were you speeding?
Do you accept you were speeding?
There is no reason they cannot use a laser gun on a bridge.
The question is not the angle per se but whether or not there is a clear and unobstructed view over a suitable distance.
They may not and there may have been obstructions but there is no reason, in principle, that this couldn't be relied upon.
A camera doesn't become less reliable because it is looking downwards rather than onside.
Can I clarify anything for you?
All the best.