1. Senior Engineer, Dept. of Transportation, Innovative Transport Solutions, 29 De Havilland Cres, Pro Park, Bldg. 1, Persequor Technopark, Pretoria 0020, South Africa (corresponding author).
2. Associate Professor, Dept. of Civil Engineering and Centre for Transport Development, Univ. of Pretoria, Private Bag X20, Hatfield 0028, South Africa.