Study May Overplay Porter Ranch Effect on Traffic Patterns : Development: City planners support Bernson’s contention that projections for the northwest Valley have been overestimated.

A controversial study probably overestimates the traffic that new development in Chatsworth and Porter Ranch would generate over the next two decades, Los Angeles city planners say, supporting Councilman Hal Bernson’s position in a bitter debate with many of his constituents.

Citing the study as evidence, anti-growth residents and rival council candidates have accused Bernson of supporting development in the area--especially the massive Porter Ranch project--that would create a plague of traffic. Bernson in turn criticized the study’s predictions as exaggerated and based on obsolete data.

Principal City Planner Robert H. Sutton said Thursday that he plans to ask Kaku Associates, the consulting firm that prepared the study, to recalculate its predictions using more recent information about community traffic patterns.

The Kaku study was based on 1984 statistics from the Southern California Assn. of Governments about community traffic patterns. At the time the study began, the 1984 information was the most recent data available, said Dick Kaku, president of the consulting firm.

However, SCAG recently released data from 1987, and projections based on those figures might give a “rosier picture” of the area’s future, Sutton said.

Sutton declined to estimate how much the results could change, but city senior traffic engineer T. K. Prime said the difference could be sizable.

“We have a general sense that it will be reduced, but we don’t have a handle on how much it will be reduced by,” Prime said.

The study--which predicted in a worst-case scenario that traffic on some streets could increase as much as 725%--is part of the environmental impact report prepared for the new Chatsworth-Porter Ranch District plan, a proposal for guidelines to govern the area’s long-range development.

The traffic portion of the environmental report studied future traffic patterns under several development scenarios.

In the worst-case scenario, it concluded that if all the land in the Chatsworth-Porter Ranch area were developed to the maximum extent allowable under the proposed district plan--called a build-out--every major street in the community would be choked.

“The transportation system cannot sustain the amount of development currently allowed under build-out of the proposed plan,” the report states. “The enormous traffic volumes in the build-out scenario argue strongly for some sort of density reductions in the plan.”

If all land in the northwest San Fernando Valley were to be fully developed under terms of the plan, the study projects, drivers would encounter a 725% increase in traffic on Chatsworth Street east of Reseda Boulevard. Afternoon peak traffic along Plummer Street east of Topanga Canyon Boulevard would increase by 680%. Other area streets would see traffic increases between 200% and 300%.

In order to handle this many cars without extensive traffic backups at intersections, the report states, streets in the area would need to be widened tremendously.

Topanga Canyon Boulevard would require 10 lanes in each direction; De Soto Avenue seven lanes in each direction; Tampa Avenue, Devonshire Street and Nordhoff Street six lanes in each direction; and Roscoe Boulevard and Winnetka Avenue, five lanes in each direction, the report states. It said the Simi Valley Freeway would also have to be widened--to seven lanes in each direction--to accommodate the projected traffic volumes.

The forecast became the center of heated debate between Bernson and many residents of the area, who seized on this worst-case scenario as evidence that strict limits must be placed on growth and that the population densities allowed under the long-range plan are too high.

But Bernson argues that the traffic projections are inaccurate because they were based on obsolete data. Furthermore, he said, it is unlikely that all the building allowed would ever occur since that would require tearing down many existing buildings and rebuilding at a higher density.

“We are not advocating or promoting this,” Bernson wrote recently in a letter to the Planning Department.

Kaku agreed Friday, saying that “total build-out of the district would require you to wipe out most industrial buildings and rebuild at a much higher density. This is just a planning exercise to give you the far end of where you stand.”

Projected market conditions for the area, Kaku said, indicate that much of the land in the community will not be built up to a higher density, and therefore traffic will not increase as much.

Kaku said he considers another scenario, the “market-driven” model, to be the most probable.

In that scenario, the report projects the greatest traffic increases would be 298% on Canoga Avenue north of Devonshire Street, 187% on Mason Avenue north of Devonshire Street and 232% on Chatsworth Street east of Topanga Canyon Boulevard. In this projection, traffic elsewhere would increase up to 180%.

Most major streets would need to be widened somewhat to provide additional through-lanes, and some dead-end streets would need to be extended to handle the extra cars, the study advised.

In a third scenario--allowing for no new buildings beyond those that have already been approved for the area and without the Porter Ranch project--community members could expect to encounter some traffic increases, though certainly not as heavy, the report said.

The no-growth scenario actually forecasts some decreases in traffic--up to 42%--on the north-south streets just south of the Simi Valley Freeway.

But afternoon peak traffic on Chatsworth Street east of Topanga Canyon Boulevard would increase up to 127%, and most other streets would see increases up to 70%.

Plugging the new SCAG data into these models will “probably result in some different conclusions” about traffic on the streets, Kaku said Friday.

The later data considers some factors not calculated into the 1984 figures, including projected changes in public transportation, car-pooling and the degree to which jobs would be located near housing, he said.

“It is hard to say the degree, but the suspicion is it will probably be less,” Kaku said.