When I have Hierarchy option enabled, I get an error "No Solution Found". Without Hierarchy, it works fine. There are 6 road classes, Hierarchy worked fine when I had Road Class 1-2 combined as Primary Highway, Road Classes 3-4 combined as Secondary Highway, and 5-6 as Local. When I changed Primary Highway from 1-2 to 1-3, rebuilt the dataset and ran the solution, I got an error. Any idea why is it happening? Thanks in Advance.
So, if I'm understanding you correctly, in the table for the Hierarchy, you have a range of numbers, but it seems one of them over lap. With using Hierarchy, they cannot overlap. It's also easier to have less classes of roads. For instance a 1 for all primary, 2 for secondary, 3 for third traveled and 4 for the least or locals. At least this is what I found out when I use Hierarchy. Hope this helps.
The values don't overlap, I don't think it is possible in ArcGIS or even logical to overlap the values. I assigned 1 to 3 values to Primary Roads instead of 1 to 2. Secondary Roads have value of 4 and Local Roads 5 to 6. I hope it makes sense now. There shouldn't be any problem with this change but not sure what is going wrong.
The dataset I am using has Road Class from 1 to 6. Currently, I am using 1 to 3 for Primary, 4 for Secondary, and 5 - 6 for Local.
Previously, I was using 1 and 2 for Primary, 3 and 4 for Secondary and 5-6 for Local but figured out that 3 also represents major roads. It was working fine until I changed Primary from 1 to 2 to 1 to 3.
Select only the streets for levels 1 to 3. Create a feature layer from the selection. Now symbolize each layer with different color and see if you can visually see some gaps between the lines
Use Nicole's suggestion, but first write out the selected level 1 to 3 lines to a new feature class, make a network on it and then run the service area for a large value with the Lines option (not polygons). Does the service area cover all the lines?