Traffic is a function of employment centers. The lower the cost of housing, the more demand there will be for it in that area. If the employment centers are not where the affordable housing is, there will be increased traffic with people just trying to get to their jobs. San Diego and Orange counties have the dominant employment centers but have very high housing costs.
Riverside County has relatively low housing costs but a significantly lower employment base.
Creating more affordable housing can be achieved by putting more housing in a given area, thereby lowering the individual cost of each unit. Increased densities are bad for traffic, bad for schools and bad for their property values. They didn't want it. Because of this opposition, the original plan has now been scrapped. Their new plan is not based on smart growth.