Started playing around with the nice object system in singleplayer. Ended making a bridge into the OOB area of the track (you do fall through the floor if you continue following the road )
Built up areas or small roads are 30MPH (50KPH) [This could be a little too slow, 40MPH or 60ishKPH seems reasonable]
Single Carriage roads or large roads with minimal buildings are 60MPH (100KPH)
Single Carriage roads with tight bends are 50MPH (80KPH)
Motorways or large back roads are 70MPH (110KPH)
Depending if the main race track is in use. Parts of it will probably vary between 75MPH (120KPH) and 100MPH (160KPH)
I actually found it difficult to maintain 70MPH with the tightness and nature of the roads so having some roads as 70mph would just be a little silly. 100MPH on roads that thin is a disaster waiting to happen, 100MPH doesn't seem practical at all unless it's on the track.