There are some requirements for the road.
Here in Tucson I use 2 different sections of the bike path which is great. In CA one day I used a path the other a little travelled road, think we saw 10 cars all day.
Ideally it's low traffic for both of ours safety. It should have a safe area to make changes to the bike/park a car(s). Ideally it's have a curve or curves in it but not something that causes you to come out of aero or brake, so no 90 or greater turns. Both of my testing courses here in Tucson are C or S shaped. It should have a bit of elevation change but not something that requires you to grind up the hill. It should be somewhere between 1.8-2.1 km in length with some room to run out on each end. Shouldn't be super rough. I think if it's a ton of broken pavement or really rough chipseal it could throw off the testing as you are moving around a ton and the bike is vibrating a ton. If it's a short rough section that is fine.
I'd reiterate safety first though.
Chung testing requires a different type course though.
Brian Stover USAT LII
Accelerate3 Coaching Insta