Infrastructure tends to be constrained and will likely limit one's speed to the speed of the slowest cyclist ahead, since there's no room to pass.

The other issue is that infrastructure doesn't take downgrades into account. So, while it can be safe to get up to speeds exceeding 20 mph (30 km/h) while in the center regular travel lane with a significant space cushion due to it's width, such speeds are hazardous with infrastructure. This means that with rolling terrain, the average speed will decrease significantly because one cannot safely travel at higher speeds on descents to make up for the time lost on ascents.