One restrainer for dogs of all sizes, in the form of a harness, is the Roadie. The Roadie attaches to the vehicle's seat belt system and includes a built in short walking lead. It was rated best in the industry by Tufts University's School of Veterinary Medicine in 2000. Crash tests evaluated the effectiveness of the product. For more information, visit http://www.ruffrider.com/.
Especially for Pets stores in Massachusetts also have a variety of safety devices for travel with pets including seatbelt attachments, harnesses, crates, carseats. Visit their website to find the location nearest you.
Links to websites with more information