To cite shp2graph in publications use:

Binbin Lu, Huabo Sun, Paul Harris, Miaozhong Xu, Martin Charlton (2018). Shp2graph: Tools to Convert a Spatial Network into an Igraph Graph in R. ISPRS International Journal of Geo-Information, 7(8), 293.

Corresponding BibTeX entry:

  @Article{,
    title = {{Shp2graph}: Tools to Convert a Spatial Network into an
      Igraph Graph in R},
    author = {{Binbin Lu} and {Huabo Sun} and {Miaozhong Xu} and {Paul
      Harris} and {Martin Charlton}},
    journal = {ISPRS International Journal of Geo-Information},
    year = {2018},
    volume = {7},
    number = {8},
    pages = {293},
    doi = {10.3390/ijgi7080293},
  }