A good idea but not relevant, practical or viable.
1/ most cars in Thailand are pickup's, and a tax loophole allows the rear seat to be classed as transport of goods, not people, as the tax and price is lower, and therefore there are no seat belts in the back.
2/ most accidents/deaths are from motorbikes, so its better to spend the money on sending police to patrol the roads and enforce the law on motorbikes and cars. Helmets, ghost riding, registration, licences and bad driving of motorbikes, cars and other transport.