Numeric speed bands "on the flat" do not work to differentiate riders of different ability in a road club because (a) riding in an even moderately tight group you can achieve much faster speeds than riding alone and (b) mismatched groups are usually able to stay together on the flat, but the group falls to bits on the climbs, often drastically so.
This is true, but it generally only happens in any meaningful way on large climbs, of which there generally aren't more than two or three on your average club run* and it is therefore feasible for riders to assemble at the top of a big climb if people drop off.
In the club I occasionally ride in there are three speed groups. Sometimes these groups might subdivide again.
There only time I've encountered a problem is with one particular dickhead who doesn't ride for months then turns up and expects to ride in the the group he always rides in (the middle one) because he's "not a shit rider".
Well, he is pretty shit and If the ride starts with a major climb (not unlikely if you live in a Cotswold valley)he drops off straight away.
We are then faced with the dilemma of wondering if he's decided to ride appropriately with the slowest group. If you make the mistake of thinking he has then 30 minutes later he appears shouting, swearing and screaming that we didn't wait for him. Absolute selfish twat.
*Club in Cotswolds, so generally hillier rides than most I'd guess.
