I drove north on 15 from San Diego to Las Vegas earlier this year on a Friday, early afternoon. I had no idea how bad it really was. It's more frustrating than anything because you end up driving a lot slower than you would for a long stretch of open road like that, most of the time around 60 mph. You just cannot pass vehicles, both lanes are jam packed with cars. Parts of this stretch do have the right side "truck lane" and that makes a huge difference on those parts with capacity, but it only lasts so long. I have not had the pleasure of driving southbound on a Sunday yet, with the northbound checkpoint close enough to Barstow there isn't a backup, but going south I have heard how bad it is. As much as I drive between Phoenix and Vegas and complain about how they need to finish up all those improvements, I would take that drive any day over the S. CAl to Vegas drive.