[OSRM-talk] "Time-to-destination" on OSRM is too short

[Spod OSM]

The time-to-destination shown on the public OSRM router seems to be  
much too short.

e.g. For the route from "Sheffield" to "Nottingham", via Chesterfield  
(UK), OSRM says it will take 42 minutes. That is not a realistic time  
at all. Even if you drive "as fast as possible" (breaking the speed  
limit on the motorway, where there are no speedcams) it is likely to  
take at least 60 minutes.

Google's routing says it will take 58 minutes, which is much nearer  
the minimum possible time. However that is still shorter than the  
actual time it takes for a normal driver in normal traffic.

Looking at the OSM data, it does look as if there is missing maxspeed  
data on some of the roads involved (but the maxspeed on the major  
length of motorway is correctly tagged), but presumably OSRM uses  
sensible scaled down defaults, relative to the way type, in that case?

Any suggestions as to how to help to get the public OSRM server to  
give more realistic times?