prior probability

Many cities around the world have installed high-tech red-light cameras at busy intersections to increase safety and raise traffic revenues — although some cities are already starting to phase-out these cameras. The debate over the legality of such cameras not only presupposes that many drivers run red lights; it also raises deep questions about the nature of law. The British legal philosopher H.L.A. Hart, for example, drew what many legal scholars now consider a crucial distinction between two types of legal observers or points of view: “external” observers and “internal” ones. So, what’s the difference, pray tell? According to Hart, external observers focus exclusively on the frequency of compliance or evasion with a given legal rule:

His view [i.e. the point of view of an “external” observer] will be like the view of one who, having observed the working of a traffic signal in a busy street for some time, limits himself to saying that when the light turns red there is a high probability that the traffic will stop.*

* See HLA Hart, The Concept of Law,” 3^rd edition (2012), p. 90.

Internal observers, by contrast, contemplate the reasons or motives for acting in conformity with legal rules. For internal observers, “the red light is not merely a sign that others will stop: they look upon it as a signal for them to stop, and so a reason for stopping in conformity to rules which make stopping when the light is red a standard of behaviour and an obligation.” (Ibid., emphasis in original.) But is this a meaningful distinction in the real world?

Consider Hart’s simple traffic signal example again, not from the perspective of an observer, but from the perspective of actual drivers out on the road. (A better example would be posted speed limits, which hardly anyone complies with in real life, but let’s stick to Professor Hart’s own example for now.) The fact of the matter is that not every person will stop at a red light or a stop sign — have you done a “rolling stop” today? (Notice,by the way, that the rate of compliance is now easy to demonstrate empirically with traffic cameras.)

Many drivers will be tempted to run the light or stop sign and may, in fact, end up doing so, especially if there is no cross-traffic and if there are no visible police cars nearby. In other words, even so-called “internal” observers, who know the reasons for acting at a red light and know that they should stop, will adopt an “external” perspective in deciding whether to evade or comply with the traffic signal. That is, they will often choose to take a calculated gamble and evade compliance if such evasion suits their interests (e.g. saving time and avoiding the hassle of stopping). Furthermore, just because a driver stops at a red light or stop sign does not mean that he or she has adopted an internal perspective. Instead, she may have decided to stop at the intersection because she thinks that the probability of detection is high, not because she accepts the red light or stop sign as a legitimate or valid reason for stopping.

In short, Professor Hart’s philosophical digression in “The Concept of Law” about internal and external points of view is no doubt interesting but ultimately irrelevant, is it not? At the end of the day, we can measure rates of compliance (or evasion, as the case may be); we can’t measure, however, one’s internal reasons for one’s decision to comply with or evade a given legal rule. In fact, shouldn’t we be especially suspect of reasons for compliance or evasion, since such reasons are often ad hoc or are offered ex post to justify one’s behavior? (Traffic signal below courtesy of Gallery of Lights.)

Yellow = speed up?