Isochrone Mapping for Trip Fees

At the moment we have the ability to insert a trip fee based on mileage, but living near large cities can be a problem when sometimes it takes an hour to drive 10 miles in one direction, and 20 minutes in another. What would be very useful is adding in the ability to set the trip fee based on travel time instead of mileage, or a combination of both (also adding in an additional fee for rush hour). Along with this, adding in trip fee transparency to the ordering page would be a huge plus for clients to be able to understand the fee.