1. Associate Research Fellow, Upper Great Plain Transportation Institute, North Dakota State Univ., NDSU Dept. 2880, P.O. Box 6050, Fargo, ND 58108-6050 (corresponding author).
2. Director, Upper Great Plain Transportation Institute, North Dakota State Univ., NDSU Dept. 2880, P.O. Box 6050, Fargo, ND 58108-6050.