Most states produce bicycle maps that show preferred routes, traffic volumes and shoulder widths. Start here:
https://www.fhwa.dot.gov/environment/bicycle_pedestrian/state_contacts.cfmLike all routes, the Pacific Coast is a mix of high and low traffic roads, wide and narrow shoulders. Every route is a mixed bag. The ACA does its best to find the safest route, but no route is completely safe. It's not hard to find good cycling roads. What is hard is connecting them all together in a contiguous route across the state.
Roads typically come in the low-traffic-no-shoulder variety, or the high-traffic-wide-shoulder variety (few states have the resources to add shoulders to low-traffic roads). I prefer the former to the latter, and I think the ACA does too, but I know that some people prefer the latter. There are a few roads that are high-traffic-no-shoulder; those are the ones you want to avoid, but that might not be possible for 100% of any route.
Most of the ACA Pacific Coast route through Washington is on low traffic, zero shoulder roads. Oregon has the best roads on the Pacific Coast. That's because Oregon actually makes an effort to promote cycling on the coast, and has added good shoulders on the southbound travel on many roads (northbound is not recommended). You can request or pick up an Oregon DOT bicycling map. It is largely the same as the ACA route. Northern California is mostly very low traffic with no shoulders. Central California is a mixed bag. While Big Sur is absolutely not to be missed, there are very few shoulders and frequent fog, but not too much traffic. Southern California is the riskiest part. While there are some very nice bike paths along the beach (especially through LA), there are also no-shoulder-high-traffic routes. Going through Malibu comes to mind.
Have fun!