After nightfall, a stretch of empty desert along the U.S. - Mexico border becomes a busy northbound highway. Those on it are headed for the southwestern U.S. state of Arizona in hopes of a job and a better life for the families they have left behind.  Primarily, they come from Mexico but there is a growing number of Central Americans.


But on the U.S. side of the border are the authorities, trying to snare these illegal immigrants before they can reach highways and towns and essentially disappear. U.S. Border Patrol spokesman Rob Daniels says the roughly 40 kilometer-long ?Tucson Sector? of the U.S. - Mexico line has kept his colleagues busy. ?In our heaviest time period, fiscal year 2000, our apprehensions in the Tucson Sector were more than 617,000. They?ve now dropped to just under 500,000 last year,? he says.


The Tucson Sector is presently the busiest crossing point between the United States and Mexico for illegal immigrants. Jose Matus, an immigration rights activist with the Coalicion Derechos Humanos in Tucson, says that?s the result of Border Patrol operations elsewhere. ?Ever since 'Operation Gatekeeper' started in ?94 in San Diego, California and then later 'Operation Hold the Line' in El Paso, Texas," he says, "we knew that they would come through Arizona because they were squeezing them into the Arizona area. No matter that the government does, people will come.?


Illegal immigration has prompted some U.S. residents to patrol the borders themselves. The idea began in San Diego in the early 1990s and has spread to Arizona. In reaction to the wave of people illegally crossing the border, the U.S. Border Patrol recently announced that it is adding hundreds of additional agents to patrol the Tucson Sector. But on the other side of the border, according to John Keeley at the Center for Immigration Studies in Washington, are Mexican government policies that he says encourage illegal immigration.


?They are aiding and abetting those crossings with things like the comic book it issued earlier this year and most recently a DVD version where they instruct Mexican nationals who are crossing illegally into the United States how to avoid detection by the U.S. government," he says, adding "These types of government policies are positively counterproductive to getting some order and controls on the border.?


Why would Mexico encourage its people to leave the country and go north? Immigrant rights activist Christian Ramirez, with the American Friends Service Committee in San Diego, outlines two significant benefits of that policy. ?Mexico enjoys this perfect social valve of disenfranchised sectors of its society leaving and making a better standard of living in the United States while at the same time they are sending remittances to Mexico. And remittances are now one of the largest forms of income for the Mexican government. So it?s a perfect scenario,? he says.


Mr. Ramirez notes that two-thirds of Mexico?s 90 million people live below the United Nations? defined poverty line. Tucson immigrant rights activist Jose Matus says that?s why these illegal border crossings will not stop. ?The hunger and the need for survival is greater than the risk of being confronted by the Border Patrol," he says. "And, their hope is always ?I?ll get away this time.


John Keeley at the Center for Immigration Studies says there are also problems on the U.S. side of the border. He says the U.S. businesses are profiting while their practices impose burdens on all Americans. ?Current immigration policy is a great deal for U.S. employers, but it?s a real raw deal for U.S. taxpayers. Profit lines are greatly helped because they?re willing to work for substandard wages and under substandard working conditions," he says, adding "Concurrently, this massive poor population comes into the United States, and all of a sudden you?re seeing state budgets explode.?


Mr. Keeley says state and local governments are burdened with the costs of illegal immigrant health care, educating their children, and providing extra policing and jails. Immigrant rights activist Christian Ramirez says the problem won?t be solved unless lawmakers see it differently. ?Congress has to look at migration from the perspective of economics. This is an issue of supply and demand. Now, more than ever our society has grown dependent on the undocumented workforce,? he says.


Many analysts say economic development in Mexico and Central America is the long-term answer ? creating good jobs at home so people don?t have to leave to survive. But such development takes time, and in some instances, strong political will. 


Meanwhile, as the sun sets each evening, the human torrent once again moves northward toward the U.S. - Mexico border.