; and (II) to improve habitat connectivity; (iii) the degree of the impact described in clause (ii); and (iv) the recommendations of the Secretary, including recommendations for further study aimed at reducing motorist collisions involving wildlife and improving habitat connectivity for terrestrial and aquatic species on the National Highway System, if any. (d) National threshold guidance The Secretary shall— (1) establish guidance, to be carried out by States on a voluntary basis, that contains a threshold for determining whether a highway shall be evaluated for potential mitigation measures to reduce wildlife-vehicle collisions and increase habitat connectivity for terrestrial and aquatic species, taking into consideration— (A) the number of wildlife-vehicle collisions on the highway that pose a human safety risk; (B) highway-related mortality and the effects of traffic on the highway on— (i) species listed as endangered species or threatened species under the Endangered Species Act of 1973 ( 16 U.S.C. 1531 et seq.); (ii) species identified by a State as species of greatest conservation need; (iii) species identified in State wildlife plans; and (iv) medium and small terrestrial and aquatic species; and (C) habitat connectivity values for terrestrial and aquatic species and the barrier effect of the highway on the movements and migrations of those species. . (2) Clerical amendment The analysis for chapter 1 of title 23, United States Code (as amended by subsection (b)(2)), is amended by inserting after the item relating to section 174 the following: 175. Wildlife-vehicle collision reduction and habitat connectivity improvement. . (d) Wildlife crossings standards Section 109(c)(2) of title 23, United States Code, is amended— (1) in subparagraph (E), by striking and at the end; (2) by redesignating subparagraph (F) as subparagraph (G)