I have used Network Analyst identify Service Area of Biorefineries in Missouri and its neighbors. I used Road shapefile downloaded from TIGER/Line (2017 TIGER/Line® Shapefiles). I do not use z.elevation and use Global Turns when running Network dataset wizard. When setup Analysis Setting, I selected Allowed U-Turns. I tried to set a simple run for Service Area analysis. However, the results are not as good as I expected (please see attached picture). The Service Area polygons are not spread out equally in directions. They are only cover the Northern part of the research area. I have changed input values such as impedance, trim polygon...
In the attached picture, the green is the road network, and the brown is the service area (results of NA). The yellow dots are Facilities.
Please, anyone, help me to find the issue.