Yes, 40 max.
One PBP year we had a control on the Brian Chapman about 16 km from the finish for the last hour or so, as some riders would be over 15kph but out of time at the finish.
We also had to change the Regs. when some riders on, IIRC, the Knotty 600, wanted 13.3kph as it was over 600km. The change was necessary to protect Organisers, as the difference would have meant hiring the hall for another day and having the Monday off work.
The limit is necessary for qualifying for PBP. In '99 a couple of chaps who'd packed on the PBP told me that the Long Flat Boring 600 wasn't suitable as a qualifier (it might be if ridden in 30h); there was also a feeling that anyone who needed 40h for an 'average' 600 (Pam's Dave's, WCW) probably shouldn't be attempting PBP.