Arizona vs New Mexico

Arizona and New Mexico share a long border and the I-10 and I-40 corridors, and the two states make opposite tradeoffs. New Mexico is stricter overall because it requires a child under 1 to ride rear-facing in the rear seat and requires infants to ride in the back, two rules Arizona does not have. Arizona's edge is the booster: it keeps a child in a car seat or booster until age 8, while New Mexico moves a child toward a seat belt at age 7. So each state is stricter for a different stage of childhood.

New Mexico requires a child under 1 to ride rear-facing in the rear seat, then steps through a restraint and a booster by age and weight. Arizona sets no rear-facing age (its restraints must meet the federal standard, but the law names no age) and has no back-seat requirement. So a parent who turns a 9-month-old forward, or seats an infant up front, is fine in Arizona but not in New Mexico. New Mexico's rear-facing floor is age 1, lower than the modern age-2 standard, but it is still a requirement Arizona lacks. Pediatricians recommend rear-facing as long as the seat allows in both.

Arizona requires a child who is at least 5 but under 8 and not more than 4 feet 9 inches to be in a booster, so the booster requirement runs to age 8. New Mexico moves a child to a restraint-or-seat-belt stage at age 7. So a 7 year old who still needs a booster in Arizona may legally use a seat belt in New Mexico. On the school-age booster years, Arizona is the stricter of the two, even though New Mexico is stricter overall.

Arizona sets a $50 civil penalty for a violation. New Mexico does not fix a single dollar figure in the same way in its restraint statute. The law that applies is the law of the state you are driving in. On an I-10 or I-40 trip, combine the two states' stronger rules: keep an infant rear-facing in the back (New Mexico's rule) and keep a child in a booster until age 8 (Arizona's rule). Pack for whichever rule is tougher and you will not get caught short crossing the state line.