To be clear, while McNamara was an advocate for seat belts, he wasn’t particularly instrumental in their development or adoption. In 1955 he introduced a paid option for Ford vehicles that included seat belts. But the option wasn’t popular with consumers, only 2% of new Fords the following year had it installed.
Saab was the first to include them as standard equipment in 1958, with Volvo introducing the modern 3-point seatbelts as standard equipment the following year. They remained unpopular in the US until after Ralph Nader’s 1965 book Unsafe At Any Speed which became a bestseller and prompted congress to pass the National Traffic & Motor Safety Act in 1966, and ultimately were made compulsory by states (presumably under a lot of lobbying pressure from insurers) starting in 1970.
Saab was the first to include them as standard equipment in 1958, with Volvo introducing the modern 3-point seatbelts as standard equipment the following year. They remained unpopular in the US until after Ralph Nader’s 1965 book Unsafe At Any Speed which became a bestseller and prompted congress to pass the National Traffic & Motor Safety Act in 1966, and ultimately were made compulsory by states (presumably under a lot of lobbying pressure from insurers) starting in 1970.