1. Public Transportation Fact Bookhttps://www.apta.com/research-technical-resources/transit-statistics/public-transportation-fact-book/
2. Diesel Technology Forum. For Earth Day 2019, the Customer is Always Righthttps://www.dieselforum.org/policyinsider/for-earth-day-2018-the-customer-is-always-right
3. Diesel Technology Forumhttps://www.dieselforum.org/news-and-resources/diesel-resource-library/diesel-resource-library
4. Diesel Technology Forum. About Clean Diesel—Why Diesel Isn’t Dirtyhttps://www.dieselforum.org/about-clean-diesel/why-diesel-isn-t-dirty