[jhj]

I live near Yellowstone in Wyoming. The park is a lot more massive than Zion, usually involving multi-hour drives to get around, and there are multiple roads in the park, all of which don't necessarily see the same levels of traffic.

There also tend to not be as many people driving around slowly gawking on the roads themselves (unlike Zion or Yosemite, say), since most of the park doesn't have crazy vista views, it's mainly a high altitude, flat-ish volcanic plateau in the middle. The specific sites along the roads will have the traffic mostly.