Interstate 5 is actually bike-legal for this stretch (Hwy. 99 is not). Bikes are allowed on the shoulder between Tracy (east of SF) and Santa Clarita (north of LA). It's relatively fast (lots of wind aid from passing trucks), but not very pleasant riding. In addition to all the noisy trucks passing by there's plenty of truck tire debris on the shoulder and as this deteriorates it leaves behind sharp steel wires that tend to puncture bike tires. I also found it to be relatively boring. The freeway right-of-way is so wide that there isn't much of visual interest nearby and it makes it seem like you're moving slower than you really are.

I prefer to take Hwy. 1 down the coast, but an inland alternative is to take McKean and Uvas Rds. south from San Jose to Gilroy, then take Hwy. 25 south through Hollister and at least to King City. From King City you can either jump on and off of Hwy. 101 as parts are bike legal (and mainly improved shoulders) while other sections have alternates; or you can take Lonoak Rd. back up to Hwy. 25 and continue past its junction with 198 where it becomes Peach Tree and then Indian Valley Rd. (there's a steep but short climb at the end of Peach Tree but you can walk it if necessary). That'll drop you down into Paso Robles and there are various routes paralleling Hwy. 101 from there down to Santa Barbara. Let me know if you'd like a specific routing suggestion for this stretch.