I suspect the bridge and the new road is being built both for security concerns as well as to simply move traffic much better as that route is a major route to get from Las Vegas into the South/East. It may also be that they are concerned with the volume of traffic going over the dam every day - I suspect it's not good for the dam!
I had to slow down quite a bit in the heavy rain...with mild showers you don't notice a problem but it was coming down so fast and hard that there was a lot of water on the road surface (it simply couldn't drain fast enough) and hydroplaning was a real concern...in those sections, which were quite long, I couldn't do over 50 and I saw a half-dozen wrecks of people who apparently didn't recognize the problem!
The MPG surprised me as well but the Z does quite well when speeds are held pretty constant and 70-80 seems to be the sweet spot for a balance between speed and mileage.
I did do some high speed passing but of course, speed wasn't what this trip was about.
It's easy to see tipple digits and the 370 practically leaps to high speed - you wouldn't think that loosing a few pounds and adding a few HP would make that much difference but as good as the 350 is, the 370 is noticeably better.