Walker, John; Pickford, Andrew: 'Afterword' (Transportation, 2018), 'Road Pricing: Technologies, economics and acceptability', Chap. 22, pp. 569-570, DOI: 10.1049/PBTR008E_ch22
IET Digital Library, https://digital-library.theiet.org/;jsessionid=529bb2pojkj08.x-iet-live-01content/books/10.1049/pbtr008e_ch22