Y'all can screen shot this, because this info is what you need if you see massive traffic on the map when going between LA and Vegas. Don't count on idiotic apps to direct you, or you'll just get yourself into hot water, as seen in the situations in the news lately.
I'll do this in order from Vegas to LA, but obviously the reverse applies when going the other direction.
Traffic which backs up in Nevada, but resolves shortly after the state line:
Take US-95 from Vegas to Searchlight, then go west on NV-164 to reconnect with the 15 in Nipton. Do this for backups which appear to be more than 30 minutes total.
Traffic which backs up in Nevada, but also continues to be very slow through Baker, CA (or even through Barstow):
Take US-95 to I-40. Take I-40 west to Barstow. You will pass near Laughlin at one point, so if you have any interest in seeing Laughlin, that's a good time to stop there (though it will require going a little out of the way).
Traffic which backs up way past the California state line, once you're already on I-15 and stuck in it:
Get off on Nipton Rd or Cima Rd (currently closed), drive south toward Cima/Kelso, through the Mojave National Preserve. These are paved roads, but very remote. If you end up on a dirt road, turn around immediately and go back to the paved road. Once you're in Cima, you will be going south on Kelso Cima Road and Kelbaker Road. Keep going south until you hit I-40, then go west. Do NOT plan to go this way if you already see traffic, but rather this is an "escape" from I-15 in this area if already stuck in traffic.
Traffic north of Barstow:
Go south on either Harvard Rd or Minneola Rd, until you reach I-40, and take it west to Barstow. These roads move quickly and have almost no cars on them. Minneola is a much shorter detour than Harvard, but it's also closer to Barstow. These two are great for avoiding Barstow-area traffic in both directions. Not quite as useful as it used to be, since they moved the agricultural inspection station to the state line. That station was previously in Yermo and frequently backed up traffic, but now it's gone and that area doesn't back up as much.
Traffic between Barstow and Victorville:
Get your kicks on Route 66. Yes, old Route 66 is the answer here, and this part of it still exists. Go west on the 58, get off on Main Street, head west, and follow Main for a long time, which also becomes known as Route 66 and National Trails Highway. Take it throug Hodge (shout out to Macon Marc) and Helendale, until you connect back to I-15 in Victorville.
Traffic between Victorville and San Bernadino (if driving to/from Ventura County, the San Fernando Valley, or West LA):
You can go through Palmdale instead of the Cajon Pass. Get off on Palmdale Rd (route 18) in Victorville, head west, staying on the road for a long time, and it will eventually become the 138, taking you through Llano, Littlerock, and Palmdale,. Turn left on Pearblossom Highway in Palmdale, and follow it to the 14 freeway. Take the 14 south, which will eventually run into I-5 in the northern San Fernando Valley. If headed to Ventura County, take the 118 west from there to Simi Valley, and go from there wherever.
There you go.
The last detour above works really well if you're driving in the afternoon from LA to Vegas, as it bypasses some of the horrendous rush hour traffic, as you're likely only to hit traffic on the 14 going to Pearblossom. But you can leave during rush hour and it still won't be terrible, if you go that way.
The Searchlight detour works great when there's a backup between Jean and the state line.
Never leave Vegas to drive to LA after 2am on a weekday morning, as you will smack right into SoCal traffic at around 5:30am or later, and be hating life.
That's a common mistake I see from people, where they think they're "beating traffic" by leaving Vegas in the middle of the night on a weekday, and then find out the bad news once they get to San Bernadino at 6am or whatever.
In general, traffic in southern California backs up going south and west in the morning, while it backs up going north and east in the afternoon/evening. This is because many people live in the outlying suburbs and commute to LA for work.
These detours have saved me tons of hours over the last 30 years driving between Vegas and LA.