In the course of a conversation about speed limits and speeding tickets, I went looking for a research study I remembered reading about. Here it is: Effects of Raising and Lowering Speed Limits, a US DOT report from 1992.

Lowering posted speed limits by as much as 20 mi/h (32 km/h), or raising speed limits by as much as 15 mi/h (24 km/h) had little effect on motorist’ speed. The majority of motorist did not drive 5 mi/h (8 km/h) above the posted speed limits when speed limits were raised, nor did they reduce their speed by 5 or 10 mi/h (8 or 16 km/h) when speed limits are lowered. Data collected at the study sites indicated that the majority of speed limits are posted below the average speed of traffic. Lowering speed limits below the 50th percentile does not reduce accidents, but does significantly increase driver violations of the speed limit. Conversely, raising the posted speed limits did not increase speeds or accidents.

Since speed limits are shown to have minimal effect on driver speeds, we can say that the principal effect of a speed limit is to arbitrarily impede the flow of traffic when a police officer happens to be present. We can also describe speeding tickets as randomly-issued fines for driving normally when one happens not to have noticed a police officer in the vicinity. Speed limit laws clearly do not reflect the values of the communities they regulate, and are thus an example of bad, anti-democratic law.

Furthermore, arbitrarily low speed limits may actually reduce traffic safety. Later in the same study:

It is interesting to note that compliance decreased when speed limits were lowered and accidents tended to increase. Conversely, when compliance improved after speed limits are raised, accidents tended to decrease.

Perhaps there is a “boy who cried wolf” effect at work here: after discovering, through repeated experience, that traffic speed limits have nothing to do with actual risks, one might easily discount the value of traffic regulations in general, some of which may actually have some rational basis.