SAN YSIDRO, Calif. (AP) _ At nightfall along the U.S.-Mexico border, hundreds of illegal aliens, wearing dark clothing and clinging to their children, dart through traffic to reach asylum - in the middle of a highway.

Twenty illegal immigrants, six of them women and children, were killed in traffic during the last 18 months when they tried to cross Interstate 5. They had hoped to travel north on the highway median.

Prodded by Hispanic activists, who said ignoring the problem was tantamount to murder, the state Department of Transportation (Caltrans) this summer started a six-month experiment to improve safety along the north-south highway.

Caltrans reduced the number of lanes from eight to four in July, expanding the median.

The result: The number of illegal immigrants traveling the median each day has more than doubled. However, no one has been killed in the month since the $88,000 experiment began, said Caltrans spokesman Kyle Nelson.

Aliens know Border Patrol policy keeps agents from arresting them within the haven of the median. Agents stay out of the median to avoid chasing immigrants into traffic, and to protect themselves.

Before the lanes were closed, an average 600 people ran out onto the median daily, said Steven P. Kean, Border Patrol spokesman. Since the median expansion, that figure has risen to 1,200 daily, he said.

''This is a bold move to take away lanes,'' Caltrans District Director Jesus Garcia said. ''But since this is the worst pedestrian accident problem in the nation, we feel it calls for some creative solutions.''

The move also helps slow traffic, which is important because illegal immigrants not familiar with U.S. traffic can misjudge the speed of cars heading toward them, U.S. Border Patrol officials say.

Mexican authorities gave expansion of the median a hearty approval.

''We do not want any more deaths on that freeway of our nationals,'' said Miguel Escobar, a spokesman for the Mexican Consulate General.

The number of immigrants apprehended along the border's San Diego sector hovers around 450,000 each year, U.S. authorities say.

Some politicians and local law enforcement officials say that instead of saving lives, the wider median invites more immigrants.

Less than 10 days after the median was expanded, U.S. Rep. Bill Lowery, R- Calif., of San Diego, introduced legislation to repair existing barriers at the most vulnerable areas of the border and add new barriers.

''I think it's a severely flawed policy,'' Lowery saiud. ''What we really need to do is to stop the illegal aliens from crossing. We need to step up enforcement to prevent them from getting onto the median in the first place.''

To that end, Caltrans this month proposed installing a wire-mesh fence in the freeway median, said department spokesman James L. Larson. It's not known yet when the fence might be built or how high it would be, he said.

Mexican officials say such a fence would be considered offensive.

''We're not disputing the right of the U.S. government to build whatever it wants on U.S. territory,'' Escobar said. ''Still, we do think that it's a factor of friction. It certainly is not evidence of friendship between the two countries.''

Caltrans also plans to experiment with lights along the darkest parts of the freeway to help motorists spot aliens crossing the road, Nelson said.

The fence and light proposals anger activists, who say the changes are designed more to help catch illegal immigrants than to protect them.

''Nothing is going to stop someone who wants to cross the border - nothing,'' said Ricardo Duenez, a political activist in San Diego and Tijuana, Mexico. ''The fence would be about politics, not safety.''