The speed limit on the I 40 is 75 mph- which translates to 85 mph as an average speed. Add to that the fact that it's a MAJOR east/west cross country route and well, it is busy. Lots of truck traffic.
The 93/95 are also very busy routes, though not as bad. Still pretty decent speeds w/ vehicles tho.
Of course I'm no help with alternate routes. I just know what I drive when I go from Phoenix to Vegas- and I would never wanna ride it!