Here's a very good idea of how traffic law enforcement actually works. While the reference is to one particular American city, the example resonates:
"During daylight hours (really around 5 am, later on Sunday, until about 11 pm, earlier on Sunday), drive at whatever speed traffic will allow. ISP [Illinois State Police] generally does not do traffic enforcement on the expressways in and around Chicago during daylight hours because a traffic stop invariably causes a significant increase in congestion and significantly increases the risk of accidents. Their resources are mainly used in this district during those hours to respond to accidents and incidents. It is thus essentially impossible to get a speeding ticket on the expressways during daylight hours. The exception is the Tri-State; typical daytime speeds on the Tri-State are 65 in the right lane [i.e. the slow lane], increasing to 85 in the left line [i.e. the passing lane], all with a posted limit of 55. Exceeding 90 may get you a ticket, but likely not. The easiest way to get a ticket, actually, is to fail to yield when a trooper pulls up behind you in the left lane and flashes at you to yield the right of way. I've seen them blow through traffic on the left at 100, 110, even 120 miles per hour.
Somewhere around 11 pm (it varies depending on the season and day of week), ISP switches to strict enforcement of the posted limit; this lasts until about 5 am. Thus, late at night, strict adherence to the posted speed limits (including the 50 and 45 reduced limits in the central part of Chicago) is required. ISP mostly snags drunks and out of staters passing through with this policy; the locals know better (which is why this policy is so effective at snagging drunks).
Once you get outside the ring of the Tri-State, the ISP has different policies, and speed enforcement becomes more common, especially in the construction zones."
In other words, the principles of speed enforcement are:
- Allow drivers to use their best judgement when it comes to posted upper limits.
- Cause no harm - don't cause traffic jams or crashes by rigid adherence to the powers vested in you wrt limits.
- Use resources to deal with real problems at times when those real problems tend to occur most.
They do not stand on principle, and limits are flexible and related to actual conditions.