Cornwall Council has subscribed to a great website which helps people to plan their journeys.
Every time that anyone wants to dig up to streets in Cornwall they need to let the Council know. The Council passes this information on to the company behind this website and they plot the information onto a map. For all the street works that are planned (ie not emergencies) you get to know about them at least a week in advance.
The mapping system is updated daily and is free to use. Unfortunately, not all councils subscribe to the system and those that don't include all of the Devon councils.