The cost of building things out of concrete, like freeways, has become very expensive. Material costs have gone up and it could go higher.

Asphalt and concrete is 80-90% sand and crushed rock. The number of quarries in San Diego is expected to decline and the county already has to import materials from outside the county. Hauling sand and rock long distances is expensive.

Historic and predicted number of mines in San Diego County. Graph created by EnviroMine.

Here is a recent report from EnviroMine to SANDAG on the issue.

According to EnviroMine, volume needs by common use are:
San Diego County - 5.4 tons per capita per year
One lane-mile of highway = > 21,000 tons
Average Hospital = 15,000 tons
Average Home = 90 tons

Missing from the list is the amount of sand needed to satisfy competing uses for sand, such as beach sands. It is noteworthy that the county is facing tax increases to put more sand on local beaches (See SANDAG Sets Stage).