If you're just concerned about making sure the slope will remain under 14 degrees, I'd recommend using several roof segments together and fit them to the general path of the driveway). Orient the roofs to the center line of the drive way, and then trim/intersect them with each other.
After you've established that the slopes work, then you can use a mesh to make a cleaner looking driveway.
A more mathematical method would be to graphically split the driveway into three segments using fills on the plan: Bottom Transition, Main Driveway, and Top Transition. Draw a polyline along the line of travel for each segment, and use the Element Information palette to get the lengths for each polyline. Establish your total rise from the road to the garage and divide it by the sum length of your polylines. If this isn't below 14 degrees, then you'll need to revise your driveway plan. If it is below 14 degrees, then you can take into account the lower slopes at the transitions to ensure that the central section doesn't exceed 14 degrees.
Scott Graham, AIA LEED BD+C
Principal | BIM Manager
Muhlenberg Greene Architects, Ltd.
Wyomissing, PA 19610
V6.5-26.0