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I did figure out a way to hack this.... Find the xml file TomTom_US_201X_QX. Within this file there are hard coded speeds for various types of RoadTypes. I edited the TomTom US Night speeds to all be 60 mph with the exception of the OffRoad roadtype.
This would be a very nice feature. In the healthcare industry driving distances are more of the standard than driving time, perhaps because miles are miles and time is dependent upon assumptions regarding speed which could be manipulated.