Go with the flow...

Driving at about 10-15 below the speed limit is not as dangerous as driving with the same difference over the limit! It's actually more dangerous!!!

Reason:

Drivers going faster than the speed limit has (statistically) better confidence and better control on the road and most importantly have better understanding of their own vehicle and the other vehicles around them. On the other hand, drivers tending to go on the speed limit and specially slower drivers (statistically) has less awareness of the surrounding objects/vehicles. Thus, I believe there must be the same fines for slow drivers too, if not stricter than the faster ones.