I have a pretty flat commute. I am usually doing 16 to 19 on my road bike on level ground. The traffic is usually light.
When the traffic gets heavier, that results in more stops at signals, but between signals my speeds tend to be higher because the other vehicles reduce my wind drag and because I put a little extra effort into keeping up with other traffic. I sometimes maintain over 25 mph for a long time if the traffic is moving just right. These are narrow-lane 4-lane roads where I ride near the center of the rightmost through lane.
When I ride around the neighborhood with my son in the trailer behind me, or when I ride my mountain bike, I go a lot slower, say 12-16.