1. Research Scientist, Institute for Transportation, Iowa State Univ., Ames, IA 50011 (corresponding author).
2. Professor, Dept. of Statistics, Iowa State Univ., Ames, IA 50011.
3. Research Scientist, Institute for Transportation and Adjunct Assistant Professor, Dept. of Civil, Construction, and Environmental Engineering, Iowa State Univ., Ames, IA 50011.