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 -
http://www.dot.gov/citizen_services/disability/disability.html
A list of federal and other resources