I believe the Portland Alpenrose velodrome closed a year or two ago (apparently a privately owned "park").
I just got an E-Mail today about a race at the Portland International Raceway. Apparently it has been going on for a while but I wasn't aware of it.
It is a flat track with a 1.96 mile loop. 4 to 23 laps depending on the category.
Not entirely straight, but designed for cars to race at high speeds.
https://www.racemondaynight.com/ (I'm not sure this is continuing).
Tuesday Night PIR (next race is Tuesday, 7/6/21, I think, and continuing into the future).
It sounds like a great idea for an off-street race venue. A few curves, but not terribly technical.
I don't know if this is common across the country. It certainly would be a way to get beginner riders racing, hopefully with less risk of major pileups.