1. Research Fellow, Transportation Research Group, Univ. of Southampton, Burgess Rd., Southampton SO16 7QF, UK (corresponding author).
2. Lecturer, Transportation Research Group, Univ. of Southampton, Burgess Rd., Southampton SO16 7QF, UK.
3. Professor, Transportation Research Group, Univ. of Southampton, Burgess Rd., Southampton SO16 7QF, UK.