I did a 20-minute FTP run this past Saturday and had a devil of a time finding an appropriate place. Since I live up in the desert, I finally settled on Willow Springs Road between Rosemond Blvd. and Tehachapi. It's a gentle but consistent grade, starts out at about 1% and works its way up to 3% by the end of the twenty minutes. One very short downhill which was part of the warmup, so it didn't come into play.
For flatter road, I'd go to SART, either upper or lower. Upper you can do any day of the week. Lower is best during a weekday when the general public usually don't use the trail.
p.s. My FTP is pretty pathetic and embarrassing. Really highlights my lack of mileage this year.