I go along with the relative term.
It truly depends on the group you're riding with.
I brought up the tail end of a 5 mile stretch of ride behind a premier local group and their 18 mph was their 'warmup'
For the group I myself rode with that same day 18 would have been the average.
I've had a local group litterly form a pcaeline behind my ford ranger and do 28/30mph down a 8 mile stretch of roadway with a strictly enforced 30mph speed limit.