A police officer making a traffic stop must have "reasonable suspicion" of a violation of law, in order for the stop to be lawful.  Not so for the checkpoint stop.  As stated by the California Supreme Court in the case of Ingersoll v. Palmer, 43 Cal. 3d 1321 (1987), the "primary purpose" of a sobriety checkpoint is "not to discover evidence of crime or to make arrests of drunk drivers but to promote public safety by deterring intoxicated persons from driving on the public streets and highways."  Id. at 1328.  Accordingly, the Court found, the lawfulness of a sobriety checkpoint stop is determined "not by the standard pertinent to traditional criminal investigative stops" – which would require that the officer have "reasonable suspicion" of a crime before performing the stop – "but rather by the standard applicable to investigative detentions and inspections conducted as part of a regulatory scheme in furtherance of an administrative purpose."  See id. 

Analogizing the DUI checkpoint to the airport security checkpoint, the Court found that the applicable test was essentially a balancing test, weighing the public interest against the rights of the motorist:

"The touchstone for all issues under the Fourth Amendment and article I, section 13 of the California Constitution is reasonableness. . . .  he federal test for determining whether a detention or seizure is justified balances the public interest served by the seizure, the degree to which the seizure advances the public interest and the severity of the interference with individual liberty. . . . California constitutional principles are based on the same considerations, i.e., balancing the governmental interests served against the intrusiveness of the detention. "Ingersoll, 43 Cal. 3d at  1328-1329. 

Balancing these competing interests, the California Supreme Court articulated guidelines for a police checkpoint that does not violate the driver's Constitutional rights, including:

• Decisions regarding the location of the roadblock and the plan for its operation are made at the supervisory level, i.e., not by the field officers;
• Officers do not stop drivers capriciously but operate the roadblock pursuant to a neutral plan requiring each vehicle, or each Xth (3rd, 5th, etc.) vehicle to be stopped;
• The location is reasonable; and proper warning signs, lighting and signals maintain safety for both motorists and officers;
• The time and duration of the checkpoint are reasonable;
• The checkpoint is marked by high visibility "including warning signs, flashing lights, adequate lighting, police vehicles and the presence of uniformed officers." This minimizes the intrusiveness of the roadblock (motorists can turn back before approaching), reassures motorists of its official nature, and promotes safety; 
• Length of stops are minimal for drivers found not to display signs of intoxication;
• The roadblock is publicized in advance, which reduces its intrusiveness and increases its deterrent effect.

Ingersoll v. Palmer, 43 Cal. 3d at  1341-1347.

Thus, under Ingersoll, police departments may, by setting up DUI checkpoints, get around the requirement that there be "reasonable suspicion" of a violation of law before stopping a moving vehicle; and can, at a checkpoint, stop motorists who are driving safely and lawfully.  However, the checkpoint must operate according to a neutral plan that minimizes intrusions into individual rights.  Motorists must be given some advance warning of the checkpoint – publication in the community, signs/flashing lights at the scene, etc. – and, as with travelers approaching security checkpoints in airports, drivers are free to choose to turn back before entering the checkpoint area, if they can do so lawfully.

Unfortunately, the reality does not match the theory.  The Ingersoll guidelines aside, many if not most police departments regard the sobriety checkpoint as a means of catching drunk drivers, not as a way to deter drunk driving.  It is therefore not surprising that many DUI checkpoints fail to advertise their presence in time to allow the motorist to turn back, and are operated more like funnel traps from which there is no escape.  But, such deficient checkpoints do, at least, give the skillful attorney an opportunity to bring a motion to suppress the evidence which, if successul, will result in dismissal of the charges.