1. Lecturer, Dept. of Civil Engineering, Univ. of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal (corresponding author). E-mail: joliveira@civil.uminho.pt
2. Lecturer, Nottingham Transportation Engineering Centre, School of Civil Engineering, Univ. of Nottingham, University Park, Nottingham NG7 2RD, UK. E-mail: nicholas.thom@nottingham.ac.uk
3. Senior Research Officer, Nottingham Transportation Engineering Centre, School of Civil Engineering, Univ. of Nottingham, University Park, Nottingham NG7 2RD, UK. E-mail: salah.zoorob@nottingham.ac.uk