It is a federal law that infants and children must ride buckled in car seats or seatbelts that have been properly installed and are used properly. Studies have demonstrated that parents often times do not position or use their car seats correctly. Many cars also have air bags for the front seat which are felt not to be safe when used with rear-facing car seats. Even a toddler in a forward facing convertible seat, a booster seat, or a lap/shoulder belt is at risk for air bag injuries. The safest place for children is in the back seat, farthest away from where a head-on crash occurs.
Massachusetts' Child Passenger Safety Law requires that infants and small children must ride in car seats until they are at least 5 years old and weigh over 40 pounds. Children who weigh more than 40 pounds, but are under 5 years old must ride in a booster seat. Once children are older than 5 years of age and are more than 40 pounds, they must wear a seatbelt that is properly adjusted. Do not place the shoulder belt under the arm or behind the back. This law applies to children riding in all types of privately owned vehicles.
For more specific information, contact your local police, state health department, or the summary of all state laws that can be found in the Occupant Protection Chart from the Safe Kids website.