Danny Birchfield drove his car into a ditch. Steve Beylund "nearly hit a stop sign while making a right hand turn into a driveway." William Bernard got his truck stuck in the river while trying to extract his boat. In his underwear.

These embarrassing incidents—the first two of which happened in North Dakota, the third in Minnesota—may not sound like the stuff of an inspiring legal battle. But all three cases, which the U.S. Supreme Court recently agreed to hear together, pose an important question about the balance between public safety and the Fourth Amendment's ban on "unreasonable searches and seizures": Under what circumstances may the state delve into a person's body, looking for evidence to use against him?

The usual answer is that the state needs a warrant issued by a judge, based on probable cause to believe the search will discover evidence of a crime. "Probable cause," a phrase used by the Fourth Amendment itself, has never been precisely defined, but it is not a very high standard. According to the Supreme Court, it may amount to no more than a "substantial chance" or a "fair probability."

Nor is getting a judge to certify probable cause much of a burden in an age of instantaneous mobile communications and electronic warrants. But police tend to take short cuts when they are available, so it is not surprising that the cops who arrested Birchfield, Beylund, and Bernard for driving under the influence (DUI) made no attempt to obtain warrants authorizing chemical testing of the alcohol in their blood.

After failing field sobriety tests, Birchfield agreed to a breath test, which indicated that he had a blood alcohol concentration (BAC) of 0.25 percent, more than three times the legal limit. But he refused to surrender his blood for testing, which in North Dakota is itself a crime: a Class B misdemeanor, punishable by a $1,000 fine and up to 30 days in jail. Birchfield conditionally pled guilty to that offense, reserving his right to appeal on constitutional grounds.

Beylund, who also did not perform well on field sobriety tests, failed to produce an adequate sample for a breath test. He heard the same "implied consent advisory" that was read to Birchfield, informing him of the penalties he could face if he did not agree to a blood test, including loss of his driver's license as well as a fine and jail. He "consented," and the test put his BAC at 0.25 percent. His license was suspended for two years based on a finding that he had driven under the influence.

Bernard, who was with two other seemingly intoxicated men the day he was arrested, insisted he had not been driving the truck that got stuck in the river. He was contradicted by several witnesses who "described him stumbling from the boat to the truck." Also he was holding the keys to the truck. Police reported that "his breath smelled of alcohol," and "he had bloodshot, watery eyes."

After Bernard refused to perform field sobriety tests, he was arrested for driving while impaired. Bernard heard Minnesota's implied consent advisory, informing him that state law "required him to take a chemical test, that refusal to take a test was a crime, and that he had a right to consult with an attorney." He nevertheless declined to take a breath test. Because he had prior drunk driving convictions, he was charged with first-degree test refusal, a felony punishable by a fine of up to $14,000 and up to seven years in prison.

All 50 states have "implied consent" laws that condition the "privilege" of driving on submission to testing in the event of a DUI arrest. In most states, test refusal does not trigger criminal penalties, but it results in license suspension and can be used as evidence against DUI defendants—a policy the Supreme Court has approved. Minnesota and North Dakota are two of 13 states that treat test refusal as a crime.

Birchfield, Beylund, and Bernard argue that threatening them with criminal penalties for refusing to submit to a warrantless search violates their Fourth Amendment rights. If a search is not valid to begin with, they say, "consent" obtained under threat of punishment cannot make it so.

In Birchfield's case, the North Dakota Supreme Court noted that he was not in fact forced to take a blood test; he was merely told what would happen should he refuse. "Consent to a chemical test," it said, "is not coerced and is not rendered involuntary merely by a law enforcement officer's reading of the implied consent advisory that accurately informs the arrestee of the consequences for refusal, including the administrative and criminal penalties, and presents the arrestee with a choice." The court concluded that in light of the threat posed by drunk drivers, "the criminal refusal statute satisfies the general reasonableness requirement of the Fourth Amendment."

Responding to Beylund's appeal of his license suspension, the same court had no trouble concluding that his acquiescence made the blood test constitutional. It also rejected his argument that making consent to chemical testing a prerequisite for a driver's license amounts to an "unconstitutional condition," requiring surrender of a constitutional right in exchange for a government benefit. "A licensed driver has a diminished expectation of privacy with respect to the enforcement of drunk driving laws," it said.

In Bernard's case, the Minnesota Supreme Court sidestepped the "implied consent" issue by ruling that chemical testing of someone charged with DUI is justified as a "search incident to arrest." The U.S. Supreme Court has said such searches are permitted without a warrant because they are aimed at finding weapons that could pose a threat to police or evidence that the arrestee might otherwise destroy. Although neither of those goals is served by testing the breath or blood of a DUI arrestee, the Minnesota court claimed that does not matter. "The Court has not applied these concerns as a limitation on the warrantless search of the body of a person validly arrested," it said, and chemical testing for alcohol falls under that heading.

The Minnesota decision in particular seems inconsistent with Supreme Court precedents. Last year, for example, the Court ruled that police may not examine the contents of an arrestee's cellphone without a warrant. Examining blood or air drawn from inside his body seems at least as intrusive, and it is equally untethered from concerns about weapons or the destruction of evidence.

The Minnesota court's distinction between searches of an arrestee's person and searches of the area near him (such as his car) likewise seems counterintuitive. "It would be perverse to suggest that a greater degree of justification is required for a search of an arrestee's property and of the area surrounding him than for a search inside the arrestee's body," Bernard's lawyers argue. The two dissenting justices in the Minnesota case complained that "the court today fundamentally departs from longstanding Fourth Amendment principles" and "nullifies the warrant requirement in nearly every drunk-driving case."

That result is hard to reconcile with Missouri v. McNeely, the 2013 case in which the Supreme Court said concerns about falling BACs do not automatically justify warrantless blood testing of DUI suspects. The implication of that decision was the police should obtain warrants for blood tests except when circumstances make it impractical. No such circumstances seem to have been present in any of these three cases, and the search-incident-to-an-arrest justification implies that a warrant is never required for a blood test, provided there is probable cause for a DUI arrest.

The Supreme Court's attitude toward the "implied consent" fiction is uncertain. In a McNeely passage frequently cited by defenders of that principle, the Court said:

States have a broad range of legal tools to enforce their drunk-driving laws and to secure BAC evidence without undertaking warrantless nonconsensual blood draws. For example, all 50 States have adopted implied consent laws that require motorists, as a condition of operating a motor vehicle within the State, to consent to BAC testing if they are arrested or otherwise detained on suspicion of a drunk-driving offense.…Such laws impose significant consequences when a motorist withdraws consent; typically the motorist's driver's license is immediately suspended or revoked, and most States allow the motorist's refusal to take a BAC test to be used as evidence against him in a subsequent criminal prosecution.

Those comments certainly make it sound as if the Court is fine with the general notion of implied consent to chemical testing, although it may nevertheless conclude that criminal penalties for refusing go too far. "This regime plainly is coercive," Bernard's lawyers note.

In truth, that description applies to all 50 states, not just the 13 that treat test refusal as a crime. "Unlike some government benefits," Bernard's lawyers observe, "driving is a necessity for millions of people who cannot earn a livelihood or participate meaningfully in society without it." They argue that states like North Dakota and Minnesota "use their monopoly on driver's licenses to coerce the surrender of constitutional rights." The same is true of all the states, including the ones where test refusal itself is not a crime, since even a driver acquitted of DUI can still lose his license for refusing to cooperate with a warrantless search.

This system is convenient for the government but contemptuous of our Fourth Amendment rights. If there is probable cause for a DUI arrest, there should also be probable cause for a blood or breath test warrant. Asking the government to take that additional step is hardly unreasonable, especially given how easily it can be accomplished nowadays. To the contrary, what's unreasonable, and therefore unconstitutional, is demanding access to the contents of someone's body without a warrant.

This article originally appeared at Forbes.com.