Originally Posted by
ofajen
The combination of supple road tires and an aerodynamic riding position on a road bike will make 12-14 mph on a road bike so slow and easy that it will be less comfortable to go that slowly than it would be to go somewhat faster, since you aren’t needing to pedal enough to unweight the saddle appreciably.
I can vouch for this. My first real ride on my road bike was 2mph faster over the same ~3 mile flat segment than on my flat-bar commuter bike with stiff puncture-resistant tires, and it felt easier too. My speeds started out in the 12-14mph ballpark. A few months of riding later (without any structured training) I rode it at 20 mph, and that's having never gone all-out on that stretch since it's at the beginning of my rides to farther destinations.