All of us are protected by the United States and Colorado State Constitutions against unreasonable tactics by police and other law enforcement. The Fourth Amendment of to the United States Constitution requires the police to be "reasonable." Centuries of case law decisions by the United States Supreme Court have slowly shaped a definition for what is considered reasonable police conduct.
When you hear the phrases "reasonable suspicion," or "probable cause," the legal basis for these concepts is rooted in the Fourth Amendment. In a DUI case, a fundamental question for your drunk driving attorney to examine is: Was there reasonable suspicion or probable cause to stop your vehicle and examine you in the first place? Probable cause can be translated to mean "the probability that you have committed or are about to commit an illegal act."
Usually the illegal act that gets a driver pulled over is a traffic infraction like speeding or swerving. The police must be able to describe in court why your actions were illegal, and establish probable cause for the judge. A DUI lawyer examines probable cause in every case. If the police officer pulled you over for an invalid reason, without probable cause, then your entire case will be thrown out.
For example, in a recent case, the police officer had pulled our client over for expired tags. The tags were in fact expired - by seven days. However, the law allows drivers a 30 day grace period in which to renew their tags. This means that it is not an illegal act to drive within 30 days of the expiration date on the tags. Because there was no law violation, there was no probable cause, and the case was dismissed both in court and at the DMV hearing.
There are numerous reasons that probable cause might not really exist. The police often misunderstand how the law is to be applied, and make mistakes that violate your Constitutional rights. In order to suppress the evidence against you, your attorney may file motions with the court, and conduct a hearing in which the police officer is cross-examined. In some cases, the District Attorney will know you have a winning motion ahead of time, and will dismiss the case before the hearing. In other cases, you may get a very favorable plea offer because the DA realizes that there is a chance he will lose at motions. Winning a motion to suppress your traffic stop wins your entire case. Because the evidence against you was obtained in violation of your Constitutional rights, the police and prosecutor will be prohibited from using it against you. With no evidence against you, the prosecutor has no choice but to dismiss the case.