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A statewide polygon GIS data layer where each polygon represents the area containing the interchange facility. For this dataset, interchanges were defined as a grade-separated junction of two or more roads where at least one road is fully access controlled and movements between roads are accomplished through straight and loop ramps. The polygon for each interchange encompasses the broad area of roadway where traffic interactions are reasonably related to the interchange, including all ramps, ramp intersections with cross streets, merging and diverging areas, acceleration and deceleration lanes, and portions of the mainline freeway that are within the general boundaries of the interchange. Each interchange was classified into a general design category, such as diamond or partial cloverleaf.
The interchange inventory includes all interchanges in North Carolina, regardless of road ownership. This interchange inventory was developed to be used primarily in Traffic Safety analysis, though other users within NCDOT as well as external users may find this dataset useful. However, all users should be aware that this dataset was constructed to be suitable for the goals of planning level crash analysis and safety screening. As such, the level of detail and accuracy may not be appropriate for more detailed analysis. For example, the borders of each interchange polygon were drawn in a manner that would ensure that a safety analysis would identify crashes within the boundary that may be related to the interchange. Further detailed safety analysis and examination of crash reports would follow this high-level identification. To allow for inaccuracies in crash geolocation or data errors, the boundaries of the interchange polygons typically encompass an area much wider than the actual paved roadway. Since the boundaries are only intended to be used in selecting crashes, the edges are roughly drawn and are not intended to convey any detailed information about the environmental, economic, auditory, or other effects of the interchange. At locations where interchange ramps intersected with surface streets, the boundaries were drawn to encompass approximately 250 feet around the ramp intersection. This distance was used as a general parameter of an “intersection influence” area for crashes, though a detailed analysis would be needed to determine which crashes would actually be logically linked to the ramp intersection.