SUVs Should Have 55MPH Speed LimitInstead of stopping NORMAL cars for speeding, cops should stop any SUVs going over 55mph. The piece of poo SUVs are a bigger danger due to their higher momentum, extended braking...
In March, you will want to head south as soon as you reasonably can do so to avoid snow in both the Rockies and Sierras.
That having been said:
US-6 is basically designed with your route in mind, but won't necessarily be fast all of the time.
If I were trying to make time, I'd take I-80 to Salt Lake and then I-15 to San Diego if the weather over the Rockies is OK. You can try I-35 or I-45 to get south to I-20 or I-10, if you decide that snow is going to be a problem.
It does snow on I-15, but usually not in March and usually not very much.
If you take I-15, ideally, you want to arrive in Las Vegas on a Tuesday or Wednesday morning and be out of there way before the weekend. The Sunday afternoon traffic leaving Las Vegas is something that you have to experience to believe. We're not talking about wimpy New York City 10 MPH traffic for a couple of exits here. If you pick your time right, you can get from Las Vegas to Barstow in an hour and a half at an average speed of about 90 MPH. If you pick the wrong time it can take you 8-10 hours.
