California Coast & Ocean provides a very thorough article on Trestles, San Onofre and the threats from the toll road.
Driving north from San Diego along the coast on Interstate 5, a traveler passes mile after mile of houses, malls, businesses, and roadways. Every now and then you glimpse the ocean off in the distance or see a grassy hillside, but otherwise you are confined to a virtual corridor of development.
Then suddenly, just north of Oceanside, the landscape opens up. To the west is the ocean, while to the east flatlands and rolling hills covered with coastal sage scrub extend as far as the eye can see, with only here and there a building or a road. Along this stretch of highway you might actually spot a hawk circling overhead, hunting for its meal among the abundant little critters in the underbrush, or perched atop a telephone pole. This is coastal southern California as it used to look before sprawl took over, and it's still here because you're in Camp Pendleton. Like many military reservations in California, this 125,000-acre Marine Corps base, owned by the Department of the Navy, contains some of the last wild land in its region; this area would likely have been developed years ago if not for the base. San Mateo Creek, one of the last coastal streams in southern California that has no dams or other man-made barriers, flows from its headwaters in the Cleveland National Forest through Camp Pendleton to meet the ocean. Southern steelhead were thought to be extinct south of Malibu Creek until they were found in San Mateo Creek in 1999.
Lots more at the site...