It's Memorial Day, which means Southern California summer traffic patterns will be setting in. School vacations and holiday weekends make for “unusual” traffic patterns for those unaccustomed to driving during those times. A quick review of common “summer driving” areas may help you save some time and frustration...
101 South/Central Coast/Santa Barbara – Typically, the 101 slows right thorugh Downtown Santa Barbara (anywhere between Cabrillo Bl and State St exits). On a holiday Monday, the Southbound commute usually slows around the curve into Santa Barbara and can sometimes stay slow on and off all the way to Ventura. Despite the lights, taking the beach route (Shoreline Dr) is usually a good alternate through Downtown. After that, it gets tricky to take a direct alternate, and even though the traffic will be slow (average 20-40 mph) through Carpenteria, it may not be worth the extra hassle of navigating local streets. In extreme cases, I have taken the scenic highway 154 which can be fun if you're not in a rush.
I-5 South/Central Valleys/Bakersfield/Santa Clarita – Usually I-5 from Northern to Southern California is the most direct and trouble-free route (and fastest – the speed limit is 70 so the average speed is 85). But all it takes is one big rig to stall out or jackknife and the road can be stopped for hours. Check with CHP if you're planning for this long route. Once you get into the Santa Clarita Valley, on a holiday Monday, traffic will often slow at Magic Mountain Parkway, sometimes even a little before, and be pretty rough all the way into the Newhall Pass. Exit any offramp and head west and you'll hit the Old Road. This is a great alternate all the way down to the 14 (it turns into San Fernando Road and you can get back on the 5 at Roxford).
I-15 South/Las Vegas/Barstow – This is one of the most miserable drives on a holiday weekend. Sometimes the drive can start getting heavy even before noon. It has taken me more than 7 hours to get home at this time, and I have heard stories of worse. There are ridiculous ways to get around this drive (going to Pahrump and then to Baker, or around side streets that are practically dirt roads for dozens of miles). If you feel comfortable following your GPS with “Allow freeway” disabled, if your estimate is less than 6 hours, it will certainly be better than taking the 15 the whole way. The worst sections are always Mountain Pass (the section just south of Stateline/Primm) and the 50-60 mile stretch between Baker and Barstow. The inspection station always messes with traffic – all traffic has to slow there, and often no one is checking anything. I highly recommend avoiding this drive anytime between 11am and 8pm on a holiday Monday.
I-10 West/Low Desert/Palm Springs/Coachella Valley – Like the drive home from Vegas, the drive back from “the river” and low desert recreation areas is almost guaranteed to be pretty frustrating. Usually it takes an extra hour to get from Palm Springs to Banning (the stretch where you see all the windmills) than normal. The peak time usually starts around 3pm and can go as late as 11pm (this is the part that frustrates most). Once you get into Banning and Beaumont there are a few side streets, but before that point, the interstate is pretty much the only way to go.
I-5 North/San Diego/Orange County – One of my most memorable calls I ever got at the KNX tipster line was from disgruntled drivers on the I-5; “I've driven this freeway for 20 years and it's never been like this!” My response was always,” Apparently you don't normally drive on the weekend.” The weekend drive on I-5 between San Clemente and Oceanside is one of the most underrated bad drives in Southern California. On holiday weekends, that factor is increased by at least two or three, and if there is an accident, it can be exponentially worse. As a good rule of thumb, taking I-15 into the Inland Empire can work, but is never fool proof. Plus, if you have to get back to Orange County, the 91 through Corona can resemble morning drive by 5pm on a monday Holiday. The best plan is to either avoid I-5 on a holiday Monday or just plan an extra 2-3 hours to your trip.
Usually leaving anytime after noon (and certainly after 3pm) on any of these “inbound” drives will prove to be just as bad as (or worse than) “prime time” during the week. These trouble areas are easy to predict but tough to detail problems, as the traffic monitoring systems are not in place as reliably as they are in more populated areas. Try as much as possible to avoid these times or to "bring a lot of patience" with you. The best resource to cite any real-time problems will be CHP.