Ecological Effects of Federal Highway Design Standards: A Network-Level Look at Habitat Fragmentation, Road Classification, and Historic Construction Practices

Our study aims to characterize the landscapes surrounding different functional classes of roads in the mountain region of North Carolina to estimate the effects of design standards governing each functional class. We investigated landscapes within three buffer distances from roads to measure landscape pattern, habitat fragmentation, and alignment characteristics. The road effect zone is known to be larger for sinuous roads, wider roads, and higher traffic volume roads. Thus, a secondary goal was to identify which geometric design characteristics, governed by Federal design policies, may influence differences in these landscapes.
This study is a first step in identifying the physical patterns that result from a) geometric road design guidelines and b) from construction and alignment technologies that contributed to road network form and pattern.
The NCDOT Roads Database was divided into 1-mile segments to create a normalizing unit of measurement for length-dependent variables like sinuosity and elevation change. Road design characteristics, either given within the NCDOT dataset or calculated in GIS, included functional class, right-of-way width, design speed, sinuosity, elevation change, traffic volume, and era of construction based on FHWA historical road-building eras. Landscape composition, topography, and elevation were calculated in GIS within three buffer sizes around each 1-mile segment. FRAGSTATS was used to compute a set of landscape metrics for each of the areas delineated by the three buffers. Metrics were selected for their robustness across studies and their cross-species generalizability. Regression analyses were used to investigate the relationships between the road and landscape characteristics for each buffer size.
Preliminary results indicate that there are differences in landscape composition among different functional classes and in the direction and magnitude of change as distance increases from the road. Results from the regression analyses are expected to confirm these preliminary estimates and enable identification of primary influences on landscape pattern and fragmentation by road classification.
This study shows that ecological considerations may be more necessary for some road classifications than others. Secondary roads, which are more likely to fall under local jurisdiction, may require different guidelines for ecological policy and planning than federal and state roads. Little right-of-way along these roads is under local jurisdictional control. Therefore, planning for roadside management and habitat connectivity requires different tools. These roads comprise over 70% of the road network regionally and have a large impact on habitat connectivity.
Future research could include the landscape and habitat impacts of anticipated changes in geometric design for automated vehicles, new construction technology, or federal policy that may alter the form and shape of only certain functional classes of roads in the network.