The best systems of law are those that began as informal rules that were formalized with some arbitrary decisions made to make people happy. The rules of the road are probably one of the best examples of this. While most of them have become dictated by the needs of law, they are based on the original principles of road use and allow the roads to be used by a variety of different forms of traffic, from pedestrians and automobiles to bikes and animals. By setting priorities and allowing for current technologies, the rules have undergone a number of revisions since they were first formalized, making them an interesting system to look at.
It may sound weird to describe them as “common sense”, but most of the rules are that way as modified by current needs. Most of the rules are based on the idea that they need to be kept relatively simple yet allow for a wide variety of situations. Consider how many decisions that a driver has to make on the road, and how many of them are simple conditionals based on the rules in place: An experienced driver can can go with the flow of traffic because most of the decisions are dependent on the situation rather than requiring actual decision making. A driver has a framework in which to make his decisions and so it is easy to use that framework to make decisions.
There is obviously the occasional glitch. The system is not prefect as it relies on human judgment and that can sometimes be imperfect. You have the rebels that purposely buck the system for their own need to rebel and those that drive while under the influence of drugs and alcohol. Combined with mechanical failures, the system does occasionally break down, requiring a lawyer to help decide the matter. A lawyer can help a person get the best possible defense, resulting in the best possible sentence. This does not mean the lawyer will get the person free with no cost, but a person has a much better chance of doing so with one than without one. Ultimately the law of the street is open to interpretation, and with a translator you can do better than without one.