Types of Construction
The FHWA design exception process can also vary based on the type of project. To understand design exceptions and the design exception process, three types of roadway construction are defined below.
New construction is defined as roadways that are built on new alignment. During the route location process, designers should be identifying corridors with sufficient width to enable full criteria to be met. It should therefore generally be easier to meet design criteria with new construction because alignments can be chosen and refined to reduce site constraints and minimize impacts. As a result, there are usually fewer design exceptions on new construction projects.
Reconstruction is defined as roadways that are rebuilt primarily along existing alignment. Reconstruction normally involves full-depth pavement replacement. Other work that would fall into the category of reconstruction would be adding lanes adjacent to an existing alignment, changing the fundamental character of the roadway (e.g., converting a two-lane highway to a multi-lane divided arterial) or reconfiguring intersections and interchanges. According to FHWA, design exceptions are required for projects involving an existing alignment or corridor for which reconstruction is proposed. Design exceptions may be more common on reconstruction projects because of additional site constraints and, in some areas, years of development and land use changes.
The term 3R stands for resurfacing, restoration, and rehabilitation projects. 3R projects typically involve pavement improvement work (short of full-depth replacement) and targeted safety improvements. 3R projects generally involve retention of the existing three-dimensional alignment. States may request approval of 3R-specific criteria for non-freeway 3R projects on the NHS, or they may use the same minimum criteria used for new construction. If 3R criteria are approved by FHWA, any of the 13 controlling criteria not meeting these values would require a design exception for a 3R project on the NHS.
For reconstruction and 3R projects, highways are often modified temporarily during construction to provide space for construction work and equipment. Because the adopted criteria are based on assumptions for vehicles traveling on finished highways and there is such a wide variety of site-specific issues within construction zones, formal design exceptions are not required for the design of work zones.