When you create a network dataset, the wizard looks at the attributes present and if it detects some time or speed information, it will add a drive time attribute as well. You can add your own and they will show up with the name you give it. For example you could have DriveTime, TravelTime, Minutes, etc as three different time based impedances and you can choose from one of them when solving.
To answer the question of where is it comming from, you can bring up the properties of the network dataset and click on the attributes tab. It will show you the name of all the attributes present in your network dataset. Click on the Evaluators button on the right side and then select the drive time attribute from the drop down at the top. The "Value" column will show where the information is comming from. It may be a field or an expression. If it is an expression then click on the first row to select it and then click on the "Evaluator Properties" button on the right to bring up the Field evalutator and you will see how the driveime expression was derived.
Jay Sandhu
