1. Annual report of PT operator BSAG/Bremen (Geschäftsbericht 2014), Bremer Straßenbahn AG BSAG, Bremen 2015 download: http://www.bsag.de/fileadmin/user_upload/redakteure/unternehmen/berichte/2015/Gesch%C3%A4ftsbericht_2015.pdf.
2. ICCT /The International Council on Clean Transportation: European Vehicle market statistics Pocketbook 2014; http://eupocketbook.theicct.org.
3. Nasdaq newsletter : Global Electric Bus Market is Expected to Grow at a CAGR of 28.0% During 2014 to 2020: Persistence Market Research - See more at: download: http://globenewswire.com/news-release/2015/06/05/742503/10137492/en/Global-Electric-Bus-Market-is-Expected-to-Grow-at-a-CAGR-of-28-0-During-2014-to-2020-Persistence-Market-Research.html.
4. UITP: Buses today and tomorrow, media background, Brussels (2010).