forcing all car buyers to pay for manufacturer installed backseat alarms is overkill and would take a few years to implement.
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right now, parents can purchase proximity alarms. the transmitter can be attached to a baby's clothing and the parent can carry the receiver or sync to a smart phone. if the predetermined distance is exceeded between the two units the parent's alarm sounds. works for kids in the car, in a store or out in a park.
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much more economical for parents to purchase proximity alarms than to force the installation costs onto all car buyers.