My club has separate speeds for different types of terrain:

D – Casual
Flat Terrain: 10–12 mph
Moderate Terrain: 8–10 mph
Hilly Terrain: 6–8 mph

C – Easy Touring
Flat Terrain: 12–14 mph
Moderate Terrain: 10–12 mph
Hilly Terrain: 8–10 mph

CC – Touring
Flat Terrain: 14–16 mph
Moderate Terrain: 12–14 mph
Hilly Terrain: 10–12 mph

B – Moderate
Flat Terrain: 16–18 mph
Moderate Terrain: 14–16 mph
Hilly Terrain: 12–14 mph

BB – Spirited
Flat Terrain: 18–20 mph
Moderate Terrain: 16–18 mph
Hilly Terrain: 14–16 mph

A – Training
Flat Terrain: 20+ mph
Moderate Terrain: 18–20 mph
Hilly Terrain: 16–18 mph

(Most of the mid-week rides are advertised as CC pace, but actually go at a BB / A pace.)