By David Daggett

A commonly misunderstood, misinterpreted and misapplied tenet of law is the level of suspicion that is required to make a routine traffic stop. Is it reasonable articulable suspicion (RAS)? Is it probable cause (PC)? Both? Neither? No matter where you look – the Supreme Court of the United States, federal opinions or Maryland appellate decisions – you can find cases citing one or the other, or oftentimes mixing the two in the same opinion. The objective of this month’s blog is to try and clear up any confusion.

Everyone agrees that a routine traffic stop, for purposes of briefly investigating whether a minor traffic offense has taken place, such as speeding, negligent driving or failing to stop at a stop sign constitutes a “seizure” within the meaning of the Fourth Amendment. See Delaware v. Prouse, 440 U.S. 648 (1979). Even though the purpose of the stop is limited and the resulting detention might be very brief and for the most part, a minor inconvenience, traffic stops are nonetheless seizures. That being said, the operative word there is “stop.” There must be a stop effectuated by the police. The Fourth Amendment isn’t implicated when the vehicle in question was already stopped. The police may certainly pull in behind a vehicle which is already stationary and approach the driver (though obviously so long as it is not merely stopped at a red light or stuck in traffic.) Approaching a stopped vehicle merely qualifies as an “accosting” and requires no level of suspicion on the part of law enforcement to do so.

The Supreme Court in Illinois v Caballes, 543 U.S. 405 (2005) held that a “routine traffic stop is a relatively brief encounter and is more analogous to a Terry stop…than a formal arrest.” The Court quoted from Knowles v. Iowa, 525 U.S. 113 (1998), quoting from Berkemer v. McCarty, 468 U.S. 420 (1984).

So what does Terry v. Ohio, 392 U.S. 1 (1968) have to do with traffic stops? Terry authorizes the police to stop and frisk an individual when there is a “reasonable suspicion that criminal activity is afoot and that the suspect is armed and dangerous and poses a threat to the officer or others.” While Illinois v. Cabellas holds that a routine traffic stop is analogous to Terry in the sense that “criminal activity” is analogous to a traffic violation, it obviously does not require a reasonable belief that the suspect be armed and dangerous. The Terry “reasonable suspicion” standard is akin to “reasonable articulable suspicion”, and is a lower standard than “probable cause.” But how much lower?

The Supreme Court tackled this question in U.S. v. Sokolow, 490 U.S. 1 (1989), holding that the officer “must be able to articulate something more than an inchoate and unparticularized suspicion or hunch. The Fourth Amendment requires some minimal level of objective justification for making the stop.” It went on to determine that “that level is considerably less than the proof of wrongdoing by a preponderance of the evidence.”

From a mathematical point of view, preponderance of the evidence can be as little as 50.1%, or, in other words, more likely than not, which is the same standard as required for probable cause. As probable cause is the standard required for arrest, clearly reasonable articulable suspicion is a lesser standard of suspicion than is probable cause. In Florida v. Royer, 460 U.S. 491 (1983), the Supreme Court held that an investigative detention based upon reasonable articulable detention must be temporary and last no longer than is necessary to effectuate the purpose of the stop. Similarly, the investigative methods employed should be the least intrusive means reasonably available to verify or dispel the officer’s suspicion in a short period of time. Reasonable suspicion of criminal activity warrants a temporary seizure for the purpose of questioning limited to the purpose of the stop. In other words, if a reasonable suspicion stop lasts too long, it can turn into an arrest and an arrest requires probable cause.

How is reasonable suspicion determined?

How is the trial judge to determine if the officer had a reasonable suspicion? The Supremes tackled this question in U.S. v. Cortez, 449 U.S. 411 (1981), ruling that “the process does not deal with hard certainties, but with probabilities. Long before the law of probabilities was articulated as such, practical people formulated certain commonsense conclusions about human behavior; jurors and factfinders are permitted to do the same.” They went on to say that “the evidence collected must be seen not in terms of library analysis by scholars, but as understood by those versed in the field of law enforcement.”

They further addressed the issue in U.S. v. Arvizu, 534 U.S. 266 (2002), holding that the process of making a reasonable suspicion determination allows officers to draw on their own experiences and specialized training to make inferences from and deductions about the cumulative information available to them that “might well elude an untrained person.” In other words: training, knowledge and experience. We’ve all heard those words before so don’t hesitate to delve into your officer’s background. The better he can explain to the court the basis of his suspicions, the greater the chance of getting past a motion to suppress.

Pretextual Stops

Everyone is familiar with Whren v. US, 116 S. Ct 1769 (1996), which authorized certain pretextual stops (Though see Charity v. State, 132 Md. App. 598 (2000) and Ferris v. State, 355 Md. 356 (1999)). The Supreme Court determined that the temporary detention of a motorist upon probable cause (their words, not mine) to believe that he has violated the traffic laws does not violate the Fourth Amendment’s prohibition against unreasonable seizures, even if a reasonable officer would not have stopped the motorist absent some additional law enforcement objective. In Whren, the Court used the term “probable cause” (though also interspersing the term reasonable suspicion), ultimately holding:

In this case we decide whether the temporary detention of a motorist who the police have probable cause to believe has committed a civil traffic violation is inconsistent with the Fourth Amendment’s prohibition against unreasonable seizures unless a reasonable officer would have been motivated to stop the car by a desire to enforce the traffic laws….


…Here the District (Trial) Court found that the officers had probable cause to believe that petitioners had violated the traffic code. That rendered the stop reasonable under the Fourth Amendment, the evidence thereby discovered admissible, and the upholding of the convictions by the Court of Appeals for the District of Columbia Circuit correct.

Here, the Court applied the probable cause standard to the stop, basing it on a motorist being stopped at a stop sign for an unreasonably long time, turning suddenly without signaling, and taking off at an unreasonable speed.


The following cases are cited (in chronological order) to point out the apparent inconsistency between reasonable articulable suspicion and probable cause when determining the level of suspicion that is required to justify a traffic stop.

Alabama v. White, 110 S. Ct. 2412 (1990)

The standard of reasonable suspicion–satisfaction of which is necessary to justify an investigatory stop of a person, consistent with the Federal Constitution’s Fourth Amendment–is less demanding than the probable cause standard for an arrest or for issuance of a search warrant under the Fourth Amendment, not only in the sense that reasonable suspicion can be established with information that is different in quantity or content than that required to establish probable cause, but also in the sense that reasonable suspicion can arise from information that is less reliable than that required to show probable cause; reasonable suspicion, like probable cause, is dependent upon both the content of information possessed by police and its degree of reliability; the quantity and the quality of information are considered in the totality of the circumstances that must be taken into account when evaluating whether there is reasonable suspicion; thus, if an informant’s tip has a relatively low degree of reliability, more information will be required to establish the requisite quantum of suspicion than would be required if the tip were more reliable; a totality-of-the-circumstances approach applies in the reasonable suspicion context as well as in the probable cause context, the only difference being the level of suspicion that must be established.

Rowe v. State, 363 Md. 424 (2001)

Rowe is best known for holding that the momentary crossing of the edge line of a roadway and later touching of that line did not amount to an unsafe lane change or unsafe entry onto the roadway, conduct prohibited by TR § 21-309, and thus, wouldn’t support the traffic stop.

It also contains a treasure trove of other legal nuggets related to this month’s topic:

  • The reasonableness of an investigative traffic seizure is evaluated under a dual inquiry: whether the officer’s action was justified at its inception, and whether it was reasonably related in scope to the circumstances which justified the interference in the first place;
  • The police may stop a vehicle when they have probable cause to believe that a traffic violation has occurred;
  • A traffic stop may also be constitutionally permissible where the officer has a reasonable belief that criminal activity is afoot (here, the officer espoused a possible DUI or “wellness check” on the driver);
  • Whether probable cause or reasonable articulable suspicion exists to justify a stop depends on the totality of the circumstances;
  • The Court of Appeals declined to consider whether the community caretaking function is applicable in Maryland, as it was not argued. However, according to Chief Judge Bell, “assuming arguendo that the community caretaking function is applicable in traffic stops in Maryland, the record in Rowe failed to show specific and articulable facts to justify the stop.”

Blasi v. State, 167 Md. 483 (2006)

“…there seems to be some imprecision as to what will justify a traffic stop—whether the officer needs probable cause to believe that a traffic offense has been committed or only a Terry v. Ohio reasonable articulable suspicion that such is the case. Courts, including the Supreme Court and this Court, have mentioned both of those standards in the context of traffic stops. The prevailing view among courts that have resolved that issue, and the view that we shall adopt, is that the appropriate minimum standard is reasonable articulable suspicion. “

State v. Williams, 401 Md. 676 (2007)

“Most courts that have determined the appropriate standard (Probable Cause or Reasonable Articulable Suspicion) that applies to a routine traffic stop, including a Whren Stop, have held that probable cause is not ordinarily required and that a traffic stop is justified under the Fourth Amendment if the officer had a reasonable articulable suspicion that a traffic law has been violated. We believe that reasonable articulable suspicion is the appropriate test for an initial traffic stop, including a Whren stop.”

Smith v. State, 182 Md. App. 444 (2008)

“…the officer has probable cause to believe that the driver has committed a traffic violation, or the officer has reasonable articulable suspicion that criminal activity may be afoot, including reasonable articulable suspicion to believe the car is being driven contrary to the laws governing the operation of motor vehicles…”

While seemingly confusing on first read (as well as on second, third and fourth readings), it appears the above paragraph stands for the following:

  • a “traffic violation” means observing something that on its face is against the law: speeding (as determined via pace or radar versus mere observation), failure to stop at a stop sign, running a red light, passing a school bus with amber lights flashing, passing on a double yellow, equipment repair order, expired or stolen tag, etc., and is justified under a probable cause standard. This allows the officer to detain the driver for the length of time required to write a traffic citation for the violation/s;
  • “laws governing the operation of motor vehicles” refers to actions that might indicate a traffic offense, but isn’t necessarily a per se violation merely based on initial observations: impaired driving, texting while driving, possibly an excessive tinting violation, etc., that requires further investigation. The standard for these types of offenses merely requires reasonable articulable suspicion. That being said, it is strongly recommended that an officer (unless having received special training) not use excessive tinting as the sole basis for a RAS stop. It’s just too risky;
  • Criminal activity afoot” is the same terminology as used in Terry v. Ohio and requires reasonable articulable suspicion.

Smith v State, 214 Md. App. 195 (2013) SAME STATE/DIFFERENT SMITH

To be reasonable, a traffic stop must be supported by “reasonable articulable suspicion to believe that the car was being driven contrary to the laws governing the operation of motor vehicles.”


While it is apparent that one can find a number of opinions employing either (or both) RAS and PC in justifying a traffic stop, a good rule of thumb when arguing a suppression motion for a traffic stop in an impaired driving case is that reasonable articulable suspicion is the level of suspicion that is required, not probable cause.

When the stop is based upon a clear cut violation of the traffic code such as speeding, stop sign and red light violations, passing on a double yellow, etc., it certainly can’t hurt to argue that the officer actually had probable cause to stop the vehicle, which is over and above reasonable suspicion.

If the suspect vehicle was already stopped on its own volition, the Fourth Amendment is not implicated and the officer can develop his RAS to detain based upon personal contact.

When initiating a traffic stop for a suspected DUI based solely upon “erratic” driving observations, the only standard of suspicion the officer can possibly develop is reasonable articulable suspicion. It will more likely be the exception rather than the rule in which an officer can justify probable cause for a DUI stop based only upon driving observations. As such, the prosecutor and officer must establish to the court as many facts as possible that justify the making of a reasonable suspicion stop, allowing the officer to draw on his or her own experiences and specialized training to make inferences from and deductions about the cumulative information available to the officer that “might well elude an untrained person.”

Listed below are some factors for the officer to take note of prior to making the stop and for the prosecutor to present to the court:

  • the amount or degree of weaving;
  • was it within the lane/across lanes/other vehicles involved;
  • straddling lane lines; swerving, problems maintaining lane position;
  • turning with a wide radius;
  • almost striking a vehicle or other object;
  • stopping issues such as stopping short or erratic braking;
  • varying speed for no apparent reason;
  • slow speed (more than 10 miles under the limit);
  • slow response to traffic signals;
  • signaling inconsistent with action;
  • the time (late at night, early morning);
  • the day of the week (Friday, Saturday, holiday, NYE, St. Patrick’s Day, Super Bowl Sunday);
  • was driver observed leaving a bar;
  • length of time and distance that erratic driving was observed;
  • crowded street or deserted street;
  • could driver be observed in vehicle? Slumped over, leaning;

Obviously, the more observations the officer makes, the stronger the case in the development of reasonable articulable suspicion. Encourage your officers to make as many observations as possible and to be sure and document them in their reports. When the case comes to trial some months later, they can always refer to their reports in order to refresh their recollection.

Special thanks go out to the American Bar Association Judicial Division and Center for Professional Development. I recently participated in a webinar entitled “The Fourth Amendment in Impaired Driving and Other Motor Vehicle Offenses: Case Law Review and Update. The Honorable Karl Grube, Senior Trial Court Judge from the State of Florida and the Honorable Earl Penrod of the Gibson Superior Court of Indiana were the moderators of that webinar. Their power point presentation was extremely helpful in shaping the direction of this blog. They are much smarter than I when it comes to Fourth Amendment issues.

David Daggett is a Transportation Safety Resource Prosecutor with the Maryland State’s Attorney’s Association. The Maryland TSRP blogs are posted with the permission of the author.