While a typical building project is limited by its footprint and size, a transportation project such as a highway is mainly a linear project which comprises of many repetitive and similar elements. A highway interchange project is less spread than a long and large straight road, but is more complex. Projects involving reconstructing or expanding existing highways are sensitive and traffic has to be managed so that the impact is as limited as possible. Construction process has to be done while minimizing public inconvenience, hence the need to plan as many details about sequencing as possible, increasing project complexity. Moreover, transportation projects are always characterized by right-of-way acquisition, a sensitive process that determines the beginning of construction. Another important challenge when dealing with a transportation project is relocation of utilities to avoid any conflicts. As a result, there are many challenges in transportation projects: design complexity, traffic control management, right-of-way acquisition, utility relocations, and also delivering projects on time and within budget.
Departments of Transportation, A&E firms, contractors and other transportation-oriented organizations need to respond to those challenges as effectively as possible. They want to avoid delay and cost overrun and come up with a successful project. They express a need for biddable and buildable projects with the smallest number of change orders and litigation [word ] as possible (Mc Manus et al 1996) . Constructability reviews are essential processes that can be implemented to make sure those challenges are addressed in a satisfying way. They identify potential planning and design flaws that may impact project performance. One of the leading technology-based tools that has started being used for this purpose is 3D and 4D CAD modeling. The ability to visualize a project in three and four dimensions gives new perspectives and develops very interesting communication-, design- and planning-oriented applications that present real benefits for constructability reviews. Even though the use of 3D/4D CAD models is increasingly common in building construction such as commercial and industrial projects, it is not extensive in transportation projects. This paper explores the value added from the implementation of 3D/4D CAD models for constructability reviews in transportation projects and aims at promoting this effective tool to gain a competitive edge.
3D/4D CAD Models for Transportation Projects
Benefits of building project modeling
Despite the scarcity of publications regarding 3D and 4D CAD models in transportation projects, there have been many studies about 3D/ 4D CAD modeling for building projects that have showed interesting benefits of the applications of such a technique for constructability reviews, which would be very useful to apply to transportation projects (Rajendran 2007)  .
3D models offer the potential to...