Bright-line rules

Why does the law fix a speed limit at an exact number instead of asking people to drive reasonably?

What we actually care about on a road is that people drive safely, so the natural rule would say: drive safely. Instead the law says thirty miles an hour — a proxy we all know to be wrong in both directions. Thirty-three on an empty dual carriageway at four in the morning is prohibited and harmless. Twenty-nine in freezing fog beside a school gate is permitted and reckless. Why write a rule you know misfires, when the accurate version is one sentence long?

Take the accurate version seriously first, because it is genuinely attractive. "Drive reasonably" fits every case, adapts to fog and to empty roads, and never produces the absurd outcomes the number produces. What does it cost?

Ask a practical question: before you set off, do you know whether you are complying? You do not. You will know when somebody else decides, after the fact — an officer at the roadside, then perhaps a magistrate. So the accurate rule has quietly transferred a decision from you to them, and in every case, not in the hard cases only.

Now ask who that decider is. If it were a single expert tribunal, unhurried and reviewable, "reasonable" might work well. But this decision is made thousands of times a day, by many different officials, at speed. Under those conditions the same conduct is judged differently by different officers, similar drivers are treated unequally, and an officer who wants to stop somebody for other reasons has a lawful excuse to hand. The vagueness that fits every case also fits every pretext.

The number solves those problems by giving up accuracy. You know before you set off. Two officers reading a radar gun reach the same answer. There is little to argue about, which means little to manipulate — and that constraint binds the official as tightly as the driver, which is the part most often missed. Frederick Schauer puts the trade squarely: a rule works by entrenching a generalisation, and entrenchment means applying it even where it is wrong. Over- and under-inclusiveness are not defects to be drafted away; they are what you bought.

There is a second saving. Louis Kaplow's framing is that rules and standards differ mainly in when the content is supplied — a rule fixes it in advance, a standard leaves it to the moment of application. Fixing it in advance is expensive once and cheap thereafter; leaving it costs nothing to enact and something in every case that follows. So the calculation turns partly on volume. A situation arising millions of times a year justifies settling it in advance; one arising rarely, in wildly various forms, does not — which is why "reasonable care" survives in negligence, where no drafter could enumerate the ways people injure each other.

Does this mean rules win wherever volume is high? Not quite, and here is the cost on the other side. A sharp line is a target: where a number carries consequences, behaviour piles up just beneath it — transactions structured to fall below a reporting threshold, a company sized just under the point where an obligation begins. And a line creates cliffs, so that a day's difference in age produces an outcome nobody would defend if asked to defend it directly.

The most common misreading is that this is an argument for rules, or that bright lines are the grown-up option and standards the woolly one. It is not. Both instruments have the same shape of cost, moved to different places: a rule pays in wrong answers at the edges, a standard pays in unpredictability and in the discretion it hands to whoever decides. Which price is worth paying is a judgement about circumstances, and above all about the applier — who they are, how many of them there are, how much pressure they are under, and how far you trust them not to use the room you have given them.

That is why the honest conclusion is a pairing rather than a ranking, and why real systems almost never use one alone. Roads have a numeric limit and a separate offence of dangerous driving, so the number handles the ordinary volume cheaply while the standard catches the twenty-nine-in-fog case it misses. Tax codes set thresholds and add anti-avoidance provisions aimed at conduct designed to sit just beneath them. Safe harbours run the pairing the other way: a general standard with a bright line inside it, guaranteeing compliance to anyone who wants certainty. The rule takes the bulk and the standard takes the residue, which is a design, not a compromise.

• Why constitutional adjudication drifted from bright-line tests toward multi-factor balancing, and what was lost.
• How anti-avoidance provisions attempt to add a standard to a rule without giving back the certainty.

Every term the collection defines is gathered in the glossary.