An overloaded van blows a tire one summer day on a quiet stretch of highway in eastern Nebraska. When police arrive, they find 23 men. All are illegal immigrants from Mexico.

The driver: A Mexican man previously convicted of smuggling who had been deported within the past year.

Authorities stop a rental van on the same road, Interstate 80, for weaving from lane to lane. It carries 41 illegal immigrants. They haven't stopped for food or bathroom breaks in nearly two days. They carry plastic jugs for their toilet. There are mounds of trash and no windows. It is mid July.

That group _ 38 Mexicans and three Guatemalans _ has been crammed together since they crossed the border in Arizona.

As illegal immigration escalates in middle America, authorities are focusing new attention on highways as popular smuggling routes _ especially I-80, a major east-west thoroughfare slicing through the farm fields of Iowa and Nebraska.

Less heavily patrolled than border state highways, these concrete arteries to the Midwest, South and East also are convenient for those who may be heading to Chicago for factory work, Georgia for fruit picking, or North Carolina for mill jobs.

Most times, Nebraska and Iowa aren't their final destinations, as was the case in both van incidents this summer.

The Immigration and Naturalization Service, Border Patrol based in Grand Forks, N.D., and Nebraska and Iowa state patrols have teamed up in recent years to conduct special roadside operations to identify illegal immigrants.

In five days in January, for instance, they seized 14 vehicles carrying 223 undocumented workers on I-80 in the Grand Island area in east-central Nebraska, says Capt. H.D. Schenck of the Nebraska State Patrol.

``The conditions most of them come in are very unsanitary,'' he said. ``In one van, there were 18 individuals. They were stacked from the floor to the ceiling, sitting on top of each other. They switched positions after so many miles.''

Police also have encountered people being smuggled with frostbitten toes, shivering in flimsy summer clothes in unheated vans during the peak of winter and mothers clutching babies in sweltering, cattle-car-like trucks in the heat of summer.

``It really is pathetic,'' says Col. Ron Tussing of the Nebraska State Patrol. ``There's not much we can do to help. You feel pretty powerless.''

Last year, 17 undocumented aliens in a van ran off a Nebraska road and smashed into a tree, leaving the driver paralyzed with a broken back.

Tussing says pulling over people on the highway makes sense because ``it's much more disruptive to them to wait until they've gotten a job. .... When you stop them on the roads, you've got a better clue how they got there, who might be responsible for smuggling them in.''

Illegal immigrants usually pay smugglers, known as coyotes, several hundreds, or in some cases, thousands of dollars to cross the border, where they may be handed over to someone else to take them to their destination.

Often, the drivers, too, are illegal.

Gerald Noland, agent in charge of the INS office in Cedar Rapids, Iowa, says some of these people are exploited by smugglers who collect part of their paychecks for years. ``It's like a loan-sharking situation,'' he says. ``They're in debt for a long time.''

``Some people try to present the image that we're the bad guys, we're out there harassing these people,'' he adds. ``Sure, the end result is they get sent back to their home country. But what kind of life is it to lead when you're an indentured servant?''

The INS office in Omaha says that in this fiscal year, federal authorities have agreed to prosecute about 20 smugglers.

Police also have discovered that illegal immigration has, at times, spawned another crime the trafficking of methamphetamine from Mexico. They believe some who deal the potent drug may be blending in among Hispanic communities, especially in areas where there are meatpacking plants.

But even with stepped-up attention, authorities concede limited resources bring limited results.

In recent months, the INS has not be able to dispatch agents to pick up some suspects stopped by the state patrol. The federal agency says if there are fewer than 15 people at the scene, it may not have staff to take them into custody _ but will alert officials in the city where they are headed.

``More often than not, we have to let them go,'' Tussing said. ``For every group of 33 or 35, there's probably three or four groups of 12 or 13 that get through. There's going to be a whole lot we do catch but a whole lot we don't.''