The following is a list of regulatory links that provide helpful information.

WC19 Information Resource: http://www.rercwts.org/WC19.html
WC19 is a voluntary industry standard for designing, testing and labeling a wheelchair that is ready to be used as a seat in a motor vehicle.

USDOT Accessibility: Equal Access to Transportation
https://www.transportation.gov/accessibility

Other federal resources

Federal Transit Administration:
https://www.transit.dot.gov/

Federal Register Notices:
https://www.transit.dot.gov/regulations-and-guidance/federal-register-notices/federal-register-notices