Hello,
Unfortunately there is no a very direct route between San Francisco and Las Vegas. In December it is probably best if you stick to the major highways. Interstate 5 south - Hwy 99 S - hwy 58 E - I-15 N. That is because if you try to take a more direct route, cutting east through Northern CA, you may run into closed roads. They are beautiful drives but they pass through the Sierra Nevada mountains. I've driven those roads in April and have found myself in near white-out conditions from a sudden snow storm. While driving the hairpin curves, I saw several cars stuck or in accidents.
I suggest planning on sticking to the main roads. But you can check the road conditions for the mountain passes before you leave San Franisco. Maybe you'll be lucky and find the weather is dry. The most likely pass you would take would be on Hwy 120, which takes you to Yosemite National Park. (Be sure to check road conditions entering and exiting.) This is where I was caught in the storm. It is of course a beautiful drive. And of course, Yosemite can be a lovely place to have lunch. There are some self-serve cafes. And there are some restaurants in the lodges. Both are over priced due to the location. I think one of the restaurants does have dress code.
http://www.nps.gov/yose/planyourvisit/conditions.htm
http://www.yosemitepark.com/Dining.aspx
Another option is to detour to Sequoia National Park. There is not a through road so this would be an out and back trip.
And you can also choose to cut through Death Valley National Park.
You can expect the drive to be about 9 hours - without stops. So doing any sightseeing along the way will be tough if you really want to do the drive in one day. Even if you don't spend much time stopping, just driving on the minor highways can add time due to lower speed limits, no-passing zones and windey roads.
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(¸. Michelle ¸.·´
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