I started in San Francisco in January 2011, hit Point Reyes NP- which has a really sweet youth hostel... then headed south on HWY 1 to Los Angeles, Octillo Wells, Joshua Tree... then out to AZ, NM, TX, etc. The weather was perfect for me... I did all the way across to Everglades FL and up to NC... I took ten weeks and only had two days of bad weather in that entire time.
I could not tell you more about North of San Francisco, though... I had the coldest winter on record in the last 60 years when I did that trip in 2011... I generally camped, unless it got below 32 deg F. I had a few really really cold rides, including getting frostbite on my face... but the traffic was not bad. I would guess the west coast traffic could be a bear from May to Sept, though.
Piglet: Because Hogs Are For Old Farts!