If you want to check fastest line, such as the one through a roundabout, then you should set the design speed to the highest permitted and create a non-editable path; do not select Generate An Editable Path.
If Autodesk Vehicle Tracking cannot complete the path at the specified design speed, the speed will be reduced to the point at which it can. Note that if the design speed has to be reduced it will be for the entire length of the path. If you wish to check individual bends within the paths you should split the path into sections.