But most of what the checkpoint turns up this night and every other night that it is active are hundreds of apparently unimpaired drivers who flash big smiles for the officers, get a reminder to buckle up, and are on their law- abiding ways in about 20 seconds.
"Hiya, buddy, how're you doing?" Officer Joseph Schrank calls out to the driver of a Chrysler New Yorker.
With his flashlight, he quickly checks the driver's eyes while an officer on the other side of the car shines a light in back.
Keeping up a friendly patter - "This is a sobriety checkpoint. You know what that's about?" - Schrank leans in closely enough to smell alcohol.
But there's no odor, and the driver is alert and able to keep up his end of this conversation, so Schrank hands him a postcard questionnaire and wishes him a good night.
Sobriety checkpoints began operating in Philadelphia in May, under a $270,000 grant from the Pennsylvania Department of Transportation.
PennDot supplies equipment, training and money to pay overtime for police to handle the assignment. The checkpoints always are done as overtime assignments so there is no conflict with other duties.
Although PennDot for a long time has paid for checkpoint programs, the practice has expanded since 1990, said Lou Rader, manager of the Pennsylvania Alcohol Highway Safety Program.
With the growing public awareness of the danger of drunken driving, the checkpoints have helped lower the state's traffic death toll, he said. In 1981, there were 943 deaths in the state from accidents involving alcohol, 681 in 1991, 596 last year.
Along with equipment and money, PennDot dispenses guidelines on how to manage the traffic safely and increase the chances that arrests can withstand legal challenges.
"We always coach the police to err on the side of caution in terms of violating anyone's personal rights," Rader said.
One key is to make sure the police have a method they stick to in stopping motorists, instead of plucking just a few drivers who seem to look suspicious.
This checkpoint was on 52d Street, for instance, because PennDot had figures that showed that stretch of road had a relatively high number of alcohol-related accidents in the last five years - 27 in one four-mile segment. Similar figures have justified checkpoints in areas that come alive at night, such as stretches of Delaware Avenue and Manayunk's Main Street.
Once the checkpoint is set up, the police have to decide whom to stop. If they can't stop every car without hopelessly snarling traffic, they pick a pattern. On 52d Street, it was stop six, wave six through.
PennDot also recommends that the checkpoints be set up so a driver can see the checkpoint and still have a chance to turn onto a different road. An escape route.
"Whenever they can - it's not required - they should give people an avenue to circumvent the sobriety checkpoint," Rader said, even though some drunks will escape arrest that way.
"The checkpoint's a deterrent tool, as opposed to an arrest tool," Rader said. "People think we're out there with a big dragnet for drunken drivers. We're hoping that people see these and become more aware. Maybe it will strike some sort of balance for their choices in the future. They'll say, 'My goodness, I could have gotten caught.' "
On 52d Street, where the Highway Patrol stopped only northbound traffic, the first traffic cones and reflective warning signs are visible from the
intersection with Lancaster Avenue, 150 yards away.
Once a driver is through the intersection, there is a smaller side street and, as a last resort, the driveway to an A-Plus market before a car is clearly inside the traffic cone corral.
All night long, some cars hesitate at Lancaster before executing hasty turns, and a few drivers suddenly remember urgent needs from A-Plus.
But hundreds of cars enter the checkpoint. Two teams of two officers check the drivers. About 10 other officers provide relief and process arrests or keep the chase car ready.
"I'm sober!" one brightly smiling young woman declares as she rolls her
window down, and since the officers concur, she's on her way in a matter of seconds.
If any drivers are upset about being delayed, they keep it to themselves, though that's not always the case.
"I had a woman call me a Nazi once. But 99.9 percent of them are nice," Schrank said.
The postcard questionnaire given to each motorist solicits reaction to the checkpoint. Lt. Edward Boothman, who is in charge of the unit, said the overwhelming majority of cards show strong support.
Though Boothman clearly wishes that the guidelines didn't make it quite so easy to avoid the checkpoints, he said the program - which also includes special roving patrols - is making a difference, beyond the number of arrests.
"The statewide average is that for every 100 cars stopped, there's one arrest for DUI. We're actually running a little below that, maybe one arrest out of 150," Boothman said.
The checkpoints also have revealed that more people are using designated drivers or being more careful about how much they drink.
The guidelines don't require the police to overlook the obvious. A Dodge minivan is being waved through the checkpoint - the last of a group of six cars getting a free pass - when the driver veers over and flattens a cone.
In a matter of moments, the driver gets a field sobriety test.
One officer parks the van at the curb. Another administers the eye test, studying the driver's pupils while he tries to follow a pen in front of his face.
Next comes balancing on one foot, followed by a heel-to-toe walking test and, finally, a hand-held device to measure alcohol content on the breath.
The hand device's results aren't accurate enough to be admissible in court, however.
Drivers who fail the field test are arrested, read their rights, and taken to the Police Administration Building at Eighth and Race Streets for a definitive breath test.
The minivan driver flunks field sobriety and becomes one of the four winners of a trip downtown during the four-hour checkpoint. Four other drivers given the test during the shift manage to pass, including the young man who drove up with a carload of friends and a big bottle of Colt .45 between his knees.
He does, however, get a ticket for having an open container of alcohol in the car.
The drama for the evening arrives in the form of a red 1983 Mercury Cougar.
Police wave for the driver to stop when the Cougar cuts through the lane of traffic cones and crosses the street, pausing briefly against the curb facing the wrong way on 52d.
Then it's off toward Fairmount Park.
A squad car takes off in pursuit.
The Cougar pauses at the Parkside Avenue traffic light, then squeals a left turn, careening down Parkside, where it hits a parked car. The two occupants make a run for it.
Minutes later, the one identified as the driver is back at the checkpoint in handcuffs, sitting on the steps of a boarded-up commercial building that has been serving as the checkpoint's holding tank, social center, and canteen for coffee and doughnuts.
Sgt. Thomas Nestel wields a laptop computer that generates paperwork for the checkpoint.
The suspect gives several names, several addresses, and several versions of who owns the Cougar.
Nestel puts down the laptop.
"Look, we're going to check it all out," Nestel says. "If you give me false information, you're going to have to start over downtown with somebody who types a lot slower than I do, and you're going to be there for three or four cheese sandwiches. You know how long that is? It's about 36 hours."
The driver settles on one name and is sent downtown.
By the time he takes a breath test, his reading is 0.08 percent, under the legal limit of 0.10 percent.
The car is not reported as stolen, but the driver faces several traffic citations.
The last field sobriety test of the shift is given at 1:45 a.m., when a man in a Ford Granada drives up with a malt liquor can in his hand and his 3-year- old son unbelted beside him.
The child is put in the back seat of Nestel's squad car. He's happily eating doughnuts while his father sits on the steps just outside.
Nestel keeps the event low-key. When a bunch of officers cluster around the father, Nestel quietly shoos them.
"We don't want the kid to have this lasting impression of all these police surrounding his father," Nestel says.
The father is given a portable phone to call the boy's grandfather, and the police get ready to drive him there.
"We're going to take you to your grandpa's house," Nestel explains to the boy. "Your dad's going to stay with us for a while."
As soon as the boy leaves, Nestel adopts a sterner tone.
"I'm going to talk to the district attorney about pressing charges for endangering the welfare of a child," he says. "There's just no excuse for that."
"Yeah, that's true," the father says. "I just dropped a buddy of mine off. He had my son."
"You should have taken your son home first."
The breath test registers 0.057 percent, well under the legal limit. No arrest.
The rest of the shift passes uneventfully, though the A-Plus continues to do a booming business.