Laws without ethical content
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A law without ethical content is one that does not proscribe or mandate an act because of the act's moral or ethical value, but for some other reason. These laws should be understood to include not only statutory law, but also case law and common law.
In general, a law may be said to have no ethical content if it represents a choice between two or more ethically equivalent options.
The existence of laws without ethical content may be used to support the central claim of legal positivism, namely that law and ethics are separable. Legal positivism states that one may disobey certain laws without doing wrong, or conversely that one may wrongfully obey certain other laws. Often contraventions fall into this category.
Traffic law 
In many countries, motor vehicles are required by law to drive in the left-hand lanes of roads with more than one lane, whilst in others they are required to drive in the right-hand lanes.
It is generally understood that whilst having such a rule provides significant obvious benefits, neither driving on the left nor driving on the right is inherently ethically superior. The law exists to protect drivers from each other, rather than to prohibit drivers from driving on an unethical side of the road.
It would not usually be considered unethical to drive on the wrong side of the road when one knows there are no other vehicles on that road, although it may well be considered foolish, but it is also usually considered good that there is a consensus as to which side of the road one should travel on, and that such an arrangement is better than having no consensus, or allowing each county, town or road to impose its own rule.
Systems of measurement 
Many countries have laws mandating a particular system of measurement for use by government bodies and commercial entities. The most common system is the metric system.
No particular system has any inherent ethical value, and in some countries they are used side-by-side. In the United States, for example, the government requires specific measure only for some uses. It does not proscribe customary units, and so in many spheres, either may be used.
In the United Kingdom, law mandates the use of the metric system by most government bodies and commercial entities. In fact the rules are more complex than this since units which are likely to confuse cannot be used: listing weights in newtons, for example, is technically correct, but not easily understood by most people.
But, the law doesn't prohibit an individual from measuring weights with such units, measuring lengths in cubits, or times in metres per c, and it would not usually be considered unethical for one to do so.
Not everyone accepts that there are laws without ethical content. Few would dispute that there is no ethical reason to prefer one side of a road or one system of units over another, but that does not necessarily mean that the law itself has no ethical value.
Although the subject matter of a law may not be an ethical issue, the law exists to form a consensus for the benefit of society, and may be understood to gain ethical value from that purpose. Legal positivism would counter that this ethical value is derived from context and not from within the law itself. If it isn't unethical to drive on the wrong side when there are no other cars on the road, then it cannot be the law itself which has ethical value.
Many, such as Robert Alexy, argue that since laws curtail individual freedoms, albeit for the benefit of society as a whole, all laws are ethically relevant. John Gardner argues that "Every legal issue, however superficially technical, is a moral issue, for its resolution inevitably has important consequences for someone."