1. Research Fellow, Dept. of Civil Engineering, Nottingham Transportation Engineering Centre, Univ. of Nottingham, University Park, Nottingham NG7 2RD, U.K. (corresponding author).
2. Research Officer, Dept. of Civil Engineering, Nottingham Transportation Engineering Centre, Univ. of Nottingham, University Park, Nottingham NG7 2RD, U.K.
3. Professor, Dept. of Civil Engineering, Nottingham Transportation Engineering Centre, Univ. of Nottingham, University Park, Nottingham NG7 2RD, U.K.