EDITORIAL : A PLANNING BREAKTHROUGH; CHANGES GET NEWHALL RANCH DEVELOPMENT OFF TO A GOOD START.

THE scaled-down version of the Newhall Ranch development that won preliminary approval Tuesday from the Los Angeles County Board of Supervisors was a pleasant surprise.

The board agreed to cut the density of the development along the Santa Clara River to 20,850 homes, a reduction of 3,500. It also required the developer, The Newhall Land and Farming Company, to set aside more than half of the 19-square-mile site for public open space.

Frankly, the concessions to the opponents made by the area's normally pro-development supervisor, Michael D. Antonovich, were more than we expected. We hope they will lead to a new era of cooperation in making Newhall Ranch an outstanding development, both for future residents and nearby communities.

Prospects for an outstanding development are excellent.

For starters, Newhall Land, which developed Valencia, has an excellent track record. And the site, between Magic Mountain and the Ventura County line, is desirable and appropriate for urban expansion.

Naturally, there will be problems.

The development, which will take 25 to 30 years to complete, will generate a lot of traffic. Much of it eventually will find its way to the already congested San Diego Freeway in the San Fernando Valley. Newhall Ranch also will put more pressure on the Santa Paula Freeway between Castaic and Santa Paula.

Many of these problems can, however, be alleviated if public officials and concerned residents work cooperatively to solve them rather than engage in endless, unproductive debates over who is at fault. The Newhall Ranch master plan provides them with a useful vehicle for doing exactly that.