1. World First as Daily Price Capping on Oyster Pre Pay Brings Benefits to Passengers. Transport for London, London, UK, February 16, 2005. https://tfl.gov.uk/info-for/media/press-releases/2005/february/world-first-as-daily-price-capping-on-oyster-pre-pay-brings-benefits-to-passengers. Accessed July 25, 2021.
2. Clipper Program Quarterly Update. Alameda-Contra Costa Transit District, Oakland, CA, 2014. https://www.actransit.org/website/uploads/board_memos/14-252%20Clipper%20Update.pdf. Accessed November 16, 2021.
3. TriMet Fares. Tri-County Metropolitan Transportation District of Oregon. https://trimet.org/fares/index.htm. Accessed July 13, 2021.