Banner for The Criminal Justice Systems: A Guide for Law Enforcement Officers and Expert Witnesses in Impaired Driving Cases

Understanding Criminal Cases

Constitutional Principles

It is critically important to understand that in our criminal justice system, unlike many others around the world, the defendant is innocent until proven guilty. This foundational assumption is a core principle established by the United States Constitution. It underscores every aspect of a criminal case. A brief review of constitutional principles may be helpful in understanding how the protection of the rights of the accused sometimes places a burden on others. The intent of this review is to help you understand why you may be called to testify more than once on the same case.

United States Constitution—Bill of Rights


Fourth Amendment:


The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.


Fifth Amendment:


No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the militia, when in actual service in time of war or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.


Sixth Amendment:


In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial, by an impartial jury of the State and district wherein the crime shall have been committed, which district shall have been previously ascertained by law, and to be informed of the nature and cause of the accusation; to be confronted with the witnesses against him; to have compulsory process for obtaining witnesses in his favor, and to have the assistance of counsel for his defense.

State Law / Preemption The United States Constitution is the supreme law of the land, and preempts (overrides or “erases”) any inconsistent State or local law. So, no State or local government can diminish the rights guaranteed by the United States Constitution. On the other hand, every State is free to adopt laws that expand the rights of individuals (suspects/defendants) beyond those guaranteed by the United States Constitution. This is true for all 50 States.

We sometimes hear news reports of decisions of the United States Supreme Court stating that certain police actions have been upheld as constitutional. However, law enforcement and prosecutors, indeed the citizenry of each State, must realize that even though the police actions passed muster under the United States Constitution, they may still run afoul of a State law as interpreted by that State’s courts and thus not be allowed by that State. On the other hand, if the United States Supreme Court issues a decision stating that a particular police action is unconstitutional under the United States Constitution, the decision is automatically binding on police everywhere, even if some State court decision or statute purports to allow the police action.


In light of the constitutional principles discussed above, it may be necessary for a witness to testify more than once in the same case. The following chart details the chronology of a criminal case and is provided to assist you in understanding why this may be necessary: