Missoula to Yuma

This is the time of the year that I tend to prefer the Interstates, because of the priority for keeping them open. Heavy traffic also helps to defeat winter weather. So I-15 to Las Vegas, then US-95, or RCMan's more detailed route being slightly shorter.

I would recommend I90 to Butte. I15 to Las Vegas. Us93 to Kingman.
I40 West to Arizona 95. Arizona 95 south to Quartzsite. Us 95 to Yuma.

Salt Lake traffic is not all that bad if you avoid Fridays and afternoons after 3 PM.

If you are sure there will be no weather then US 93 all the way to Kingman would be OK. Then on to Yuma as I described above. Be careful as you can have ice covered roads in Nevada during the winter even when the weather is clear and sunny.

But if there is weather I prefer the Interstates.

US93 is (mostly) 2 lane, but I wouldn't describe it as 'secondary'. Here in Washington they are just a diligent about plowing US2 as I90.

The only pass on US93 is Lost Trail on the state line. You'll be over it the first day, so if you leave when the conditions are good, you'll be fine. If conditions are bad on Lost Trail, they are likely to be bad on other routes out of Missoula. I90 to the west crosses 2 passes on the way to Washington. To the east it meets I15 at a high point. I15 has a pass on the state line - though it isn't quite as high as 93, and broader.

Once in Twin Falls you still have the choice between staying on 93 or taking I84 to I15 in Utah. I15 v US93 has been discussed many times. People like 93, but acknowledge that it is more remote. I15 has SLC traffic.

Yes, I-90 to I-15 to Vegas then US95 to Yuma.

I-90 to Butte, then get on I-15 all the way to Vegas. I've driven this numerous times - watch the weather.

Check out the new GPS units from Garmin or Magellan. They now can link to your smart phone and show weather conditions for your route directly on the GPS.

Having traveled in cold country a lot I would recommend staying on interstates unless you are familiar with secondary highways and watching weather reports closely. Several years ago I was heading south from Missoula and got directions from a local person and almost got stranded in a snow storm. Neither that person nor I was aware of of a storm coming in. Be careful. Interstates are monitored and plowed regularly, secondary hwys are not.

Should have added on my previous post that we typically do this route the first week of January. We are careful to look at the weather ahead of us and try to pick a 2 day window of opportunity to travel. We make Jackpot the first day and are in Vegas on the afternoon of the 2nd day. You appear to be seasoned travelers so I'm betting you will have no problems.

From Missoula I would drop straight south to Twin Falls, then 93 south to Jackpot, NV. on to Ely then take Hwy 318 to Vegas. From there you only have a couple of choices but at that point you are out of cold weather and can lolligag if you wish. We are from Helena so our route to Twin Falls is different but from there on we have traveled this route many times.

I too would go I5, US93, if I remember correctly, has a seasonal time limit on it and you may end up back tracking to get around it. US93 is great during the rest of the year, but not during the winter months. Another option is to go I15, somewhat farther but also a whole lot easier and safer. Plus be sure to have tire chains, some passes require them. We were born and raised in Deer Lodge and have driven 93 quite a few times. While heading south earlier in the year, 93 is a great choice.