Thanks a ton for taking a look at my data. I know that vast majority of the roads have 0 minutes but the roads surrounding the county centroids have some value in the minutes field. If you just try with 5 minutes then you will see that most of the county centroids have a service area around them except for Caldwell and Hayes. Hayes is not on the network so lets ignore it. For Caldwell, it is generating the service area across whole of Texas. And as you pointed out, is it because of the edges with zero minutes?? I just saw a few of them near to the County Centroid named Caldwell. Do you know of any road network dataset for Texas which will have the minutes for all the edges?? Then I can use that to get the service area with time as impedence.
Once again, thanks for your inputs.....:)