The primary function of the ruling in this case is really twofold. First, it clarifies the validity test established in Brown v Texas, and it reaffirms the reasoning of a previous checkpoint case involving illegal immigration.
Brown v Texas established what is usually termed a Three-pronged Test. This test defined the prongs as the Subjective Severity of the Intrusion, the efficacy of the measure in question, and the Degree of State Interest in the desired result of the given measure. The Supreme Court of Michigan applied this test to the Checkpoints mentioned in Michigan v Sitz. The Michigan Court focused primarily on the Severity of Intrusion, and the Efficacy measures, while tacitly conceding the State's important interest in the desired aim of reducing drunk driving. The Michigan Court claimed that the violation of privacy required for this measure was severe, and that the measure was ineffective. Thus, they ruled the procedure inconsistent with US CON. Amendments 4 and 14.
SCOTUS noted in their ruling that the question of efficacy is not within the purview of the courts. The Opinion stated in no uncertain terms that the ideal procedure shouuld be left to the determination of the relevant experts, rather than the whim of the courts. This is probably wise.
The second bone of contention between SCOTUS and SCMI is is the difference of interpretation of the question of Severity. SCMI posits that the intrusion is severe and extensive. SCOTUS disagreed, reasoning that the the degree of intrusion, ie violation of privacy, is much less severe than SCMI would have them believe. SCOTUS cites teh average stop length for the checkpoints in question as being around 25 seconds. Further, a checkpoint of this nature contains more than one officer, usually roadblocks, and significant advance notice. As a result, the checkpoint is less likely to cause undue emotional stress, compared to the routine roving patrol stop conducted by a single officer on a dark road at night. As a result of this difference, SCOTUS reversed the lower Court's Opinion, reinstating the checkpoint measure as valid under the 4th and 14th Amendments.