Next time you see a mobile radar unit, notice when it starts to flash and blink. They're calibrated to warn you about speeding as soon as you cross 10% over the posted speed limit. And if there's a cop hiding behind such a unit, he won't get notified until you cross the 10% threshold.
The mobile radar units are not required to be regularly calibrated as they are typically used only as a reminder that motorists might be speeding.
The tripod mounted lidar units used by the police at speed traps are accurate to within 1% provided they are calibrated correctly.
But here's the problem with that. When drivers see 90km/h, they speed up to 110km/h. If they see 100km/h, they speed up to 120km/h. Posted speed limits are what they are for a very good reason, with safety and fuel conservation in mind.
On the surface that would seem to make sense. If drivers feel they can get away with 10 over, then they drive 10 over. However that's not generally the case.
Regardless of the limit posted at the side of the road, the driver's speed will generally be influenced by the road design, hills, curves, number of intersections, number of other cars, weather conditions, etc.
So what actually happens when you raise the posted speed limit from 90 to 100 or 100 to 120 is you get a higher percentage of drivers voluntarily complying with the posted limit.
When the posted limit is more in-line with what the majority of drivers view to be a safe speed for the road, you see fewer drivers pushing the limit and travelling 10-20km/hr over.
In short, raising the speed limit by 10-20km/hr does not automatically result in everyone "speeding" by another 10-20km/hr.
Interesting piece - a speed limit study back in 2003 actually saw a reduction in collisions at test sites where they raised the limits:
"The results of the analysis are shown in Table 8 for the Phase I sites and in Table 9 for the Phase II sites. Based on the analysis, it appears that raising the limit from 90 km/h to 100 km/h resulted in a 12.9 percent reduction in crashes at the sites where speed limits were raised. The Phase II sites experienced an 8.6 percent reduction in total crashes. Both reductions are
Why the reduction in the number of collisions? The higher speed limit more accurately reflected the average travel speeds of motorists. This resulted in more consistent vehicle flow and less speed differential between vehicles.
In short, the slower drivers sped up to the new speed limit, but the previously "speeding" drivers didn't speed up by another 10km/hr.