Just a quick & dirty 5,10,15,30,60 minutes bike isocrhrone map of Bologna using road & cyclepath network available on Openstreetmap It clearly show from the central main square how the Bologna center areas (the one surronded by the Bologna circular Vialis)have approx a 5 minutes radius...
Also an alternative version from the Bolognina main cross with 5,10,15,30 minutes isocrhones
This is based and using the OSMnx[1] library
[1] Boeing, G. 2017. “OSMnx: New Methods for Acquiring, Constructing, Analyzing, and Visualizing Complex Street Networks.” Computers, Environment and Urban Systems 65, 126-139. doi:10.1016/j.compenvurbsys.2017.05.004