1. Graduate Research Assistant, Virginia Tech Transportation Institute, Blacksburg, VA 24061 (corresponding author).
2. Professor, Dept. of Civil and Environmental Engineering, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061.
3. Assistant Professor, Dept. of Mathematics, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061.
4. Lead Scientist, Western Research Institute, Laramie, WY 82072.
5. Graduate Research Assistant, Virginia Tech Transportation Institute, Blacksburg, VA 24061.