1. Dept. of Civil, Architectural and Environmental Engineering, Univ. of Texas at Austin, 301 E Dean Keeton St., Austin, TX 78712.
2. Associate Professor, Dept. of Civil, Architectural and Environmental Engineering, Univ. of Texas at Austin, 301 E Dean Keeton St., Austin, TX 78712.
3. Director, Center for Transportation Research, Associate Professor, Dept. of Civil, Architectural and Environmental Engineering, Univ. of Texas at Austin, 301 E Dean Keeton St., Austin, TX 78712 (corresponding author).