There's what the law says. Then there's the law that gets applied. Why do we view the former so differently to the latter?

So - quick pop quiz. What is the maximum speed you can travel on New Zealand's open road without incurring legal sanction?

If your answer was anything other than "it depends", you failed. Because, as the police's recent Queen's Birthday speeding blitz indicates, the speed at which you may drive without getting in trouble with the law jumps around all over the place.

Sure, there are signs that say "100" scattered along the edge of our highways and byways - some of them on ridiculously twisting stretches of road that only experienced rally drivers really should travel on at more than 70 km/h. But you'd be silly to assume that those signs indicate the speed you are permitted to drive at.

First up, all posted limits are superseded by a general requirement to travel at a "safe" speed. Traveling "at a speed or in a manner which, having regard to all the circumstances, is or might be dangerous to the public or to a person" breaches a legal duty. And it doesn't matter if the speed you are traveling at is lower than the posted maximum for that stretch of road - driving at any speed deemed for any reason to be "dangerous" breaches the law and can be punished. What then constitutes a "dangerous" speed often gets decided in retrospect; you get involved in a crash (or near crash), and ipso facto your speed was dangerous.

Of course, the contrary doesn't apply. If you drive at a speed that is greater than the posted maximum, then you technically commit an offence even if your speed is not "dangerous" having regard to all the circumstances. (I guess the theory is that as it is unsafe for anyone to go over 100 km/h in any conditions on any road, you should face potential legal sanction for doing so - but I'm not sure this theory stands up to a cruise along the Ida Valley-Omakau Road on a clear summer's day.)

But here's where things get somewhat tricky from a jurisprudential standpoint. Yes, the formal legal rule is clear: "drive at more than 100 km/h (or 80/70/50 depending on the speed zone) and you commit an offence." However, everyone knows that you can exceed that speed by some margin without running any risk of being pulled over by the police and sanctioned.

How do we know that? Because the police themselves tell us what their enforcement policy is. There is a standard 10% tolerance given to speeding drivers, meaning that tickets hardly ever get issued to people going less than 110 km/h. Consequently, 110 km/h has become the de facto open road speed limit.

(This near-total elision of the "enforced" limit with the "legal" limit is evidenced by a story my mother told me about a friend of hers who complained bitterly about the police ticketing her for traveling at 115 km/h, as she was "only going 5 km/h over the limit.")

However, if it is the case that the police's announced enforcement practice effectively determines the speed people may drive at, then isn't the consequence of their Queen's Birthday announcement (as well as earlier announcements like this one) a change in the law by press release?

I'll grant that the police's changed policy has no formal implications for the legal rule, as would be the case if the police announced they will start ticketing anyone driving at 95 km/h on the open road. You may, conditions always permitting, lawfully travel at 100 km/h on the open road, irrespective of anything the police say about that. A completely risk-averse driver can thus avoid potential legal liability by keeping their speedometer beneath that mark.

But many (most?) of us are not completely risk-averse when it comes to our speed. We drive up to the line of what we can get away with, not what the paper rule says. Meaning that on Thursday last week (and then again on the following Tuesday) we were able to drive at 109 km/h without fear of sanction, but Friday through Monday we only could go at 104 km/h.

Now, I accept there were perfectly good reasons for the police wanting to lower the average driving speed over the holiday weekend. I'm not a "it's all about revenue" conspiracy theorist, nor do I deny the basic laws of Newtonian physics. And who could argue with the weekend's outcome (although the link between the police's policy shift and that outcome are disputed by some)?

What is interesting, however, is the apparent ease with which the police can change the effective rules on speed, compared to the convoluted process that would have to be followed were the formal rules involved. A proposal to lower the legal, on the books, speed limit  from 100 km/h to 95 km/h would necessitate all sorts of consultation and public notice, stretching over months. The rule change would then need to be made by the Governor General in Council, before being officially Gazetted. And at the end, the police might announce the 10 km/h tolerance remains, meaning an effective speed limit of 105 km/h.

Compare this to how readily the effective speed limit can be changed to 105 km/h by the police themselves. The police decide that the holiday road toll needs to come down, and with an operational decision and media release they hive 5 km/h off the speed at which people can drive before the law will intervene. And no-one really bats an eyelid at this.

This issue of enforcement discretion is not unique to road transport, of course. The gap between "what the law says" and "how the law is applied" exists in a huge number of areas. Take the smacking of children, for example, or the rules governing the use of money at election time. Or, in the UK context, experimentation around how to deal with cannabis users.

What we don't see much of, however, is interest from the legal academy about how those sorts of decisions actually create the legal regime that people live by. Give us a statute, regulation or court case, and we'll fill pages of prose on its implications. But a policy decision by an enforcement agency, no matter how far-reaching, sails by us unremarked.

I don't know why that is. But I'm sure it can't be a healthy approach to the law.