A controversial state law could make it difficult to build highways in Riverside County, and it is likely to accelerate the transition from a housing market dominated by suburban single-family homes to one characterized more by condos and townhouses, officials said Wednesday.

Those are some likely local effects of Senate Bill 375, legislation signed into law last year that ties land-use and transportation planning to global warming.

Adopted two years after California passed a landmark law calling for a substantial reduction in greenhouse gas emissions linked to climate change, the bill by Senate President Pro Tem Darrell Steinberg, D-Sacramento, took aim at carbon emissions of passenger cars.

The Steinberg law has stirred much concern among city and county planners around the state, and particularly in fast-growing Riverside County. Ninety local officials turned out for a three-hour workshop on the topic at Lake Elsinore's baseball stadium to explore the ramifications.

Hasan Ikhrata, executive director for the six-county Southern California Association of Governments, warned that some state officials are interpreting the law as meaning no more highways should be built. And that has implications for western Riverside County, where transportation officials hope to build a 32-mile, east-west freeway ---- the Mid County Parkway ---- from south Corona to Hemet.

That sentiment could spur follow-up legislation to withhold state funding for new roads, Ikhrata said.

Anne Mayer, executive director for the Riverside County Transportation Commission, said that doesn't doom future projects, but it does mean freeway improvements will have to accommodate more than the automobile. The freeway of the future will have to be more like Interstate 15 in northern San Diego County, where express lanes are reserved for buses and car pools, in addition to solo commuters.

Mayer said her agency is looking to build similar lanes on I-15 north of Lake Elsinore and on Highway 91 in Corona. Those kinds of lanes will be key to developing a "bus rapid transit" system that can compete with the car.

"We have to have the backbone system for these buses to run on ... so they are actually rapid," she said.

While the new law doesn't bar road building, it requires regional transportation plans ---- blueprints for highway, rail and bus improvements in urban areas ---- to factor in the effects of projects on greenhouse gas emissions. It mandates that those plans be accompanied by "sustainable community strategies" that provide road maps for reaching regional emissions targets.

The Southern California Association of Governments writes transportation plans for Riverside County, and the next one is due by June 30, 2012.

By Sept. 30, 2010, the California Air Resources Board is supposed to give the association an emissions target for Riverside, Orange, Los Angeles, Ventura, San Bernardino and Imperial counties.

Rick Bishop, executive director for the Western Riverside Council of Governments, a local subgroup of the association, said those community strategies will be crafted with much help from local officials.

Besides placing a growing emphasis on public transportation, the strategies are expected to call for so-called "smart growth" development that stresses building condominiums and townhouses, and placing housing next to train stations and bus routes, employment centers and shopping centers to reduce driving.

It was clear from the workshop, said Menifee Planning Director Carmen Cave, that those kinds of features will have to be common in her new city of 48 square miles. And that is going to be a challenge because those things aren't popular with many Menifee residents, Cave said.

Menifee is writing its general plan, a local blueprint for development, and expects to finish in the spring of 2011, she said.

"We're fortunate because we're starting fresh," Cave said, after the meeting.

Many local officials view the new law as anything but fortunate.

"A lot of people, when the bill first came out, were freaking out," Bishop said.

But Bishop said demographic trends toward smaller families ---- more single parents, more retirees and fewer couples with children ---- suggest the region should be building more compact housing, anyway. And with 1 million more people expected to come to western Riverside County between now and 2035, the area needs a more diverse mix of housing to accommodate all of them, he said.

"I see more opportunity in Senate Bill 375 than conflict," Bishop said.

One key opportunity, said John Husing, a regional economist, is to begin laying the foundation for a high-paying, white-collar Riverside County job base after settling for so many years with an economy built on the blue-collar industries of construction, manufacturing and warehouse distribution. It is those white-collar jobs regional leaders believe will trigger the county's transition from bedroom community to job center.

Call staff writer Dave Downey at 951-676-4315, ext. 2623.