It is flat here where I live. The biggest hills on my 12.5-mile each way commute are highway/railroad overpasses. I ride that 12.5 miles in 42-45 minutes, door-to-door. That makes for an average speed of ~16-18mph, including the time spent at traffic lights where my speed is zero. My normal riding speed is closer to 19-20mph. If on a longer ride of 30 miles or more, I gear it down a bit and still maintain the same cadence for a 16-17 mph riding speed. My solo-ride 100-mile average riding speed is 15-16mph.
I don't use any GPS app or cycle-mounted speed sensor - I use rural roadside mileposts and my watch. 3:00 for one mile is 20mph, 3:10 is ~19mph, 3:20 is 18mph 3:30 is 17.1mph, 3:40 is 16.3mph. My 'goal' is to do my commute at faster than 18mph for the first nine miles (3:20/mi pace), then slow down to (3:40/mi pace) to cool-down for the rest of the ride...