The River Mountain Trail at the east end of the valley in Henderson is about as low traffic (none) as you can get
https://rivermountainstrail.org/ but right now with temperatures above 105 degrees, you will not want to ride it after about 10am. It's 35 miles long. There are over 250 miles of trails, MUPS, and marked bike lanes across the valley. So many, that it is hard to recommend just one. Also lots of the streets are very wide compared to much of the rest of the US so even if a street is 45 mph you may be able to ride on it quite comfortably. The RTC has a brand new bike map for Southern Nevada. You can pick up a copy at any bike shop or use the one that is online
https://www.rtcsnv.com/wp-content/up...keBrochure.pdf. I don't do off-road so can't help with gravel trails.